Which sentence is correct?
“Women live longer and have fewer illnesses than men, proving that women are the strongest sex”
“Women live longer and have fewer illnesses than men, which proves that women are the strongest sex”
The first sentence is correct.
“Which” is a relative pronoun, and so it must refer to a noun.
Correct: I saw many lichens, which are a kind of fungus.
“Which” refers to the noun “lichens”.
In the second sentence, “which” is used to refer to a clause (“Women live longer and have fewer illnesses than men”) This is a common misuse of “which”.
“Which” can absolutely refer to a clause, as in the example “He threw out my food, which made me angry.”
Sorry @bodangles but that sentence is incorrect.
It’s really not.
http://dictionary.cambridge.org/us/grammar/british-grammar/relative-clauses-referring-to-a-whole-sentence
http://www.grammar-quizzes.com/clauses-9.html
https://www.ego4u.com/en/cram-up/grammar/relative-clauses
That’s the whole point of relative clauses.
Shouldn’t it be “stronger” in both of those sentences, as opposed to “strongest” since it’s a comparison of only two?
@bodangles
from your reference http://dictionary.cambridge.org/us/grammar/british-grammar/relative-clauses-referring-to-a-whole-sentence
" We often use these clauses in informal speaking to express an opinion or evaluation"
The SAT is not a test of informal spoken English. Of course we use “which” in informal spoken English to refer to a whole clause, even though this is not acceptable in formal writing. This is exactly why this error is put on the WRITING section of the SAT. You need a grammar reference for FORMAL WRITTEN American English for the SAT. You have to be careful about online sources that do not clearly distinguish between informal and formal usage.
Yes you are right, it should be “stronger” so both sentences are wrong.
Fine, I concede. I maintain that I have never been marked down for using a relative clause to refer to another clause, including on the SAT (800 and 780 writing scores). It’s like saying contractions are ungrammatical.
I found the question online, and the answer key says it’s neither of the answers the OP gave: https://books.google.com/books?id=6peu9mBKw0cC&pg=PA543&lpg=PA543&dq=women+live+longer+and+have+fewer+illnesses+than+men,+proving+that+women+are+the+strongest+sex&source=bl&ots=HKBKVOQkdx&sig=VsvyfzGB1YeuK1ghI1mC2bARKoE&hl=en&sa=X&ved=0ahUKEwiDm5aQkv3KAhWGDSwKHYUMAtsQ6AEIITAC#v=onepage&q=women%20live%20longer%20and%20have%20fewer%20illnesses%20than%20men%2C%20proving%20that%20women%20are%20the%20strongest%20sex&f=false
Crappy link but should bring you to the right point in the book.
I couldn’t open your link. What was the correct answer choice for the OP’s original question?
I agree the answer choices the OP gave were both wrong because they had “strongest” instead of “stronger”.
Can you give any official SAT questions in which “which” is used to refer to a whole clause in the correct answer choice?
I have never seen one.
I have seen plenty of questions in which “which” is used to refer to a clause in an incorrect answer choice.
For example: OG Test 3 Section 10 Question 7 choice E
"Their flight was missed, which eventually caused the bride’s parents to run etc. " (Answer choice incorrect)
OG Test 7 Section 10 Question 4 choice B
The information age has ushered children into a global society, which is the cause of educators lamenting a lack of texts that explain the diversity of cultures. (Answer choice incorrect)
also
OG Test 8 Section 4 Question 9 Choice B (incorrect)
OG Test 8 Section 10 Question 12 Choice C (incorrect)
Probably you were able to avoid making errors on your tests because “which” referring to a clause generally occurs on the SAT together with other errors, and you must have picked up on these other errors. But I would be really, really surprised if you could come up with that usage in a correct answer choice. This is not SAT usage.
“Which” to refer to a clause is not really like a contraction. It is more like the use of “that” as a relative pronoun for people: an ungrammatical usage that is so common that some authorities think it is acceptable.
The question from the link, transcribed:
Women live longer and have fewer illnesses than men, which proves that women are the strongest sex.
A. as is
B. which proves that women are the stronger sex
C. facts which prove that women are the stronger sex
D. proving that women are the strongest sex
E. a proof that women are the stronger sex
Answer key says C.
C: “facts which prove”: “which” refers to “facts” (a noun, not a clause). This is good.
The problem with this question is that we’re evaluating questions about the SAT using non-CB materials. A question like the one in the OP is unlikely to ever be on the SAT.
I’m glad the correct answer also fixed the strongest/stronger issue. 
Glad there was not an option for “proving that women are the strongER sex” because I think that would be a better sentence than C, which [referring to a noun :-)] is the best choice of the five.