I had met him ______ year ago

Option A: This would be grammatically correct and the most natural choice in standard English, indicating a singular, indefinite time period one year prior.

Option B: "An" is used before vowel sounds, and "year" starts with a consonant sound.

Option C: This could work if "the year" refers to a specific year (e.g., "the year 2023"), but "ago" makes it awkward unless clarified (e.g., "the year before"). It’s less natural here.

Therefore, Option A is the correct answer.