The correct answer is D. No article.
The word "letter" is a common noun, which means it does not refer to a specific letter. In this case, the speaker is not referring to a particular letter, but rather to the general concept of a letter. Therefore, no article is needed.
The other options are incorrect because they all use articles. The article "a" is used before singular common nouns that refer to a general class of things. The article "an" is used before singular common nouns that begin with a vowel sound. The article "the" is used before singular common nouns that refer to a specific thing.
In this case, the speaker is not referring to a general class of letters or to a specific letter. Therefore, no article is needed.
