The correct answer is D. no article.
The word “stamp” is a common noun, which means that it does not refer to a specific stamp. In this case, the speaker is asking a general question about whether or not the listener collects stamps. Therefore, there is no need to use an article.
The article “a” is used before singular common nouns that refer to a specific thing. For example, “I bought a stamp.”
The article “an” is used before singular common nouns that begin with a vowel sound. For example, “I bought an apple.”
The article “the” is used before singular common nouns that refer to a specific thing that has already been mentioned. For example, “I bought the stamp that you gave me.”
In this case, the speaker is not referring to a specific stamp, so there is no need to use any article.