Several critics have claimed that any contemporary poet who writes for

Train yourself to read the argument critically i.e. identify gaps while you read.

Several critics have claimed that any contemporary poet who writes formal poetry—poetry that is rhymed and metered—is performing a politically conservative act.
Critical say that formal poetry writing is a politically conservative act.

This is plainly false.
Conclusion: Critics are wrong.

Consider Molly Peacock and Marilyn Hacker, two contemporary poets whose poetry is almost exclusively formal and yet who are themselves politically progressive feminists.
These two poets write formal poetry but are politically progressive feminists.

I see the gap. The premise says that the poets are progressive. But format poetry writing is a conservative act. So until and unless I can say that anyone who is a progressive cannot commit a conservative act, this needn't be false.

We need the conclusion to be logically followed. So we should be given that anyone who is progressive cannot commit a conservative act.

(A) No one who is a feminist is also politically conservative.
Irrelevant. We need to link the kind of person with the kind of act.

(B) No poet who writes unrhymed or unmetered poetry is politically conservative.
Irrelevant. We don't know anything about poets who write non formal poetry.

(C) No one who is politically progressive is capable of performing a politically conservative act.
Correct.

(D) Anyone who sometimes writes poetry that is not politically conservative never writes poetry that is politically conservative.
We don't know anything about poets who write non conservative poetry.

(E) The content of a poet’s work, not the work’s form, is the most decisive factor in determining what political consequences, if any, the work will have.
Irrelevant. Doesn't link act to the person.

Answer (C)