My father _________ swim very fast when he was young.

Correct option is C

The correct word to use here is “could,” as it refers to an ability in the past. "Can" is used for present ability, while "could" is used for abilities in the past.
Information Booster:
1. Can: Present ability (e.g., He can swim well).
2. Could: Past ability (e.g., She could sing beautifully).
3. May: Used for permission or possibility.
4. Must: Indicates obligation or necessity.
5. Understanding these modal verbs helps with correct tense usage.