Read the given sentence and choose the correct modal auxiliaries to fill in the blanks.
When I was in my twenties, I ______ lift heavy objects.

The correct answer is: Could

Both options b and c are in the present tense, while the given sentence talks about something that was a possibility in the past, thus they are eliminated. If "could" is used, it refers to an ability that a person generally had in the past, which the sentence is talking about. Hence option a is the correct answer.