Identify the grammatical error in the following sentence:

You can avoid annual (A) / account fees by registering (B) / for a savings plan (C) / to the brokerage firm (D) .

The correct option is D.

Explanation:
The given sentence is divided into four parts:
• (A) You can avoid annual
• (B) account fees by registering
• (C) for a savings plan
• (D) to the brokerage firm .

The grammatical error lies in part (D) of the sentence. The preposition "to" is incorrectly used. When referring to enrolling or registering with an institution, organization, or company, the correct preposition to use is "with".

Correct Sentence:
"You can avoid annual account fees by registering for a savings plan with the brokerage firm."