AIM GMAT wrote:Many writers of modern English have acquired careless habits that damage the clarity of their prose, but these habits can be broken if they are willing to take the necessary trouble.

(A) but these habits can be broken
(B) but these habits are breakable
(C) but they can break these habits
(D) which can be broken
(E) except that can be broken

Issues:

Active Vs Passive:

GMAT prefers ACTIVE voice over passive voice.

Example:

these habits can be broken. Question is "who will break these habits?" If we don't know the agent of the verb break, we may go for passive. Since we know who should break these habits, we should go with the agent, they, the writers.

--> This eliminates A and B.

Which in D refers to their prose. D changes the intended meaning of the sentence. Eliminate D.

I E, that refers to what? That is ambiguous in the sense that has more than one potential antecedent. Besides, that is used without COMMA. In E, that has been used after COMMA.

Answer is C.