The correct answer is D. no article.
The word “guide” is a common noun, which means it refers to any member of a group of people or things. In this case, the guide is not a specific guide, but rather any guide who is willing to help the speaker. Therefore, we do not need to use an article.
The other options are incorrect because they all use articles. The article “a” is used before singular common nouns that refer to a specific person or thing. 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 that has already been mentioned or is known to the reader or listener.
In this case, the guide is not a specific guide, but rather any guide who is willing to help the speaker. Therefore, we do not need to use an article.