Step 1: Rule.
The correct usage is "agree to a proposal/suggestion/plan" and "agree with a person."
Step 2: Checking options.
(A) at — incorrect usage.
(B) to — correct: "agreed to my suggestion."
(C) for — incorrect preposition.
(D) with — would apply to people, not suggestions.
Step 3: Conclusion.
The right answer is (B) to.