Correct options.
Mr. thapa is university professor.(a/an/the/ nothing)
​

a

Explanation:

There are three (3) main articles used in English language and these are;

1. An: it is a word that is mainly used before words beginning with any of the vowel letters (a, e, i, o and u). It's an indefinite article used for modifying non-specific (unspecific) nouns.

2. A: it is mainly used with consonant sounds or letters. It's an indefinite article used for modifying non-specific (unspecific) nouns.

3. The: it is a definite article used before nouns or superlative adjectives to indicate uniqueness.

In this scenario, the most appropriate article to use is "a" because it describes the position held by Thapa.

Mr. Thapa is a university professor.