The 'letter' having been mentioned for the first time and known only to the speaker, makes option C incorrect. Since the noun is not specific, it can be any letter, an indefinite article must be used. Option A being an indefinite article, which comes before a noun beginning with a consonant, is the correct answer here. Option B being an indefinite article for nouns beginning with a vowel is the incorrect option. An article is required in the statement since we are not referring to the letter here in the general sense, hence Option D is incorrect.