You can avoid monthly (A) /bank fees by signing up (B) / for a free checking account (C) / to the local credit union (D) .

The correct option is D.

Explanation:

In English grammar, prepositions must correctly correspond to the verbs or nouns they modify.

In part (D), the preposition phrase "to the local credit union" is used after "signing up for a free checking account". However, when indicating joining or becoming a member of an institution or organization like a credit union, the appropriate preposition is "with" or "at" (e.g., "signing up with a credit union" or "at a credit union").

Therefore, the error lies in part (D).

Corrected Sentence:

"You can avoid monthly bank fees by signing up for a free checking account at (or with) the local credit union."