ACT English

How do I solve such questions in ACT?
Are there any strategies that I must know?

Passage

The Navajo code remained classified after the war
that was later used, along with codes made from other
29
American Indian languages, in the Korean Conflict and the
Vietnam War. Now that the Navajo code is no longer used,
the code talkers, whose secret work saved American lives,
30
can finally receive public recognition for their actions.

F. NO CHANGE
G. hush-hush actions
H. concealed, hidden efforts
J. doings, kept under wraps,

Thankyou so much

What is the error in the passage that the question is talking about?

The answer is F, the most concise answer.

If you look at G,H, and J they all make the sentence wordy when only two words “secret work” can do.