The correct answer is: D. no article
The word “friend” is a common noun, which means it does not refer to a specific person. In this case, the speaker is not referring to a particular friend, but rather to the general concept of friendship. 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 person or thing.
In this case, the speaker is not referring to a specific person or thing, so the article “the” is not appropriate. The speaker is also not referring to a general class of things, so the articles “a” and “an” are also not appropriate. Therefore, the correct answer is no article.