You can avoid monthly (A)/bank fees by signing up (B)/ for a free checking account (C)/ to the local credit union (D).
Read the sentence to find out whether there is any grammatical or idiomatic error in it. The error, if any, will be in one part of the sentence. The corresponding letter of that part is the answer. If the given sentence is correct as it is, then choose option ‘No error’ as the answer response. (Ignore errors of punctuation, if any).

The correct option is D.

Detailed Explanation:

Let's analyze the sentence part by part to check for grammatical and idiomatic correctness:

1. Part (A): "You can avoid monthly" - This part correctly uses the modal verb "can" followed by the base verb "avoid" and the adjective "monthly".

2. Part (B): "bank fees by signing up" - The noun phrase "bank fees" serves as the object, and the prepositional phrase "by signing up" correctly indicates the means or method of avoiding fees.

3. Part (C): "for a free checking account" - The phrasal verb "sign up" takes the preposition "for" when referring to a service, account, or event (e.g., "sign up for an account"). Thus, this part is grammatically correct.

4. Part (D): "to the local credit union" - This part contains an idiomatic error with the preposition. When indicating membership or location connection with an organization or institution such as a credit union or bank, the correct preposition to use after "account" is "at" or "with", not "to".

Corrected Sentence:

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

Therefore, the error lies in part D.