(H.S Incoming Senior).
I was planning on talking about my favorite subject as history and intended on minoring in Classics in my college personal questions (UC), but I have not decided what my main major would be.
Can I declare myself as undecided even though I talked about history in my college essay?
Thanks 
Yes, you can select Undeclared as your major. Just an FYI about going in as Undeclared at the UC’s. If you are leaning towards a highly competitive major such as Engineering/CS, Biological Sciences, Psychology etc… trying to switch into these majors later after being accepted can be problematic and many times almost impossible. Just make sure you know the risks as Undeclared and check each UC’s policy about changing majors.
The same can be problematic sometimes when talking about business majors.