The correct answer is option 'D' - "a".

Explanation:
In this sentence, the phrase "university student" is a common noun phrase. When we use a common noun phrase to talk about someone or something in a general sense, we use the indefinite article "a" before the noun phrase.

The indefinite article "a" is used before singular countable nouns when the noun phrase is nonspecific and not previously mentioned. It is used to refer to any one of a group of people or things. In this case, "university student" refers to any one of the many university students, without specifying a particular student or university.

By using the indefinite article "a" in the sentence, we understand that he is one of many university students, but we don't know which university he attends or any specific details about him. The article "a" indicates that he is a member of a larger group without specifying any further details.

Using the definite article "the" would imply that there is a specific university student being referred to, one that has been previously mentioned or is known to the speaker and the listener. However, in this sentence, there is no prior mention or specific context given, so "the" should not be used.

The articles "an" and "any" are not appropriate choices in this sentence.

- "An" is used before words that start with a vowel sound. Since "university" starts with a consonant sound, "an" is not correct in this case.
- "Any" is used in questions and negative sentences to refer to a choice from a group of people or things. Since the sentence is not a question or negative sentence, "any" is not the correct choice.

Therefore, the correct answer is option 'D' - "a".