Question: Collecting old coins and paper notes (is, are, was, were) one of my father's hobbies when he retired.
Answer: Collecting old coins and paper notes was one of my father's hobbies when he retired.
Step-by-step explanation:
If we were to answer is or are, it would mean present tense, but this question is talking about past tense.
Even if "were" is past tense, it is incorrect because it is plural and the sentence is singular. Therefore, making the answer "was".