He is proficient ___ math.

The correct preposition to use with 'proficient' when referring to a subject or area of skill is 'in'. Therefore, 'He is proficient in math' is the grammatically correct phrase.

• A. In: This is correct. 'Proficient in' means skilled or expert in a particular field or activity.
• B. With: 'Proficient with' is typically used when referring to tools or people (e.g., proficient with a computer, proficient with children), not subjects.
• C. At: 'Proficient at' is used for specific tasks or activities (e.g., proficient at playing chess), but 'in' is preferred for broader subjects.
• D. Off: This preposition is entirely incorrect in this context.