Grammar questions test a small set of rules repeatedly. Subject-verb agreement is first: a singular subject takes a singular verb, and phrases like "one of the students" remain singular.
Tense consistency is second. "Since" and "for" with a period of time signal the perfect continuous: "He has been working here since 2019." Mixing tenses within a sentence is the most common error planted in exam questions.
Articles follow sound, not spelling: "an hour" because the h is silent, but "a university" because it begins with a y sound.