_____ were classmates in the university.

Analyze the question The blank is the subject of the verb "were", so the sentence needs subject pronouns (I, he, she, we, they), not object pronouns (me, him, her, us, them).

Explain the concept

• Subject pronouns (I) come before the verb.
• Object pronouns (me) come after verbs or prepositions.
• When a compound subject has "I", good English places the other person first out of politeness: "his sister and I", not "I and his sister".

Solution

• B. "me and his sister": uses "me", an object pronoun, as subject. Wrong.
• C. "I and his sister": grammatically correct, but it breaks the politeness convention of putting yourself last. It is not the best choice.
• D. "His sister and me": again uses "me" as subject. Wrong.
• A. "his sister and I": uses the subject pronoun "I", with the correct order.

A small point: option A begins with lowercase "his" because the blank starts the sentence. That is only a formatting quirk of the question, and it does not change the grammar.

Answer: A. his sister and I

Exam tip: To test a compound subject, remove the other person. You would say "I was a classmate", never "me was a classmate". So use "I", and put it last: "his sister and I".