Fill in the blank with appropriate option.When I was young, I _ run very fast.

Explanation

'Could' is the correct verb to use as it denotes an ability that existed in the past, fitting well with the past tense 'was' in the sentence. The speaker is referring to a capability they had when they were younger. The other options are incorrect because 'can' refers to present ability, 'could have' suggests a missed opportunity or hypothetical past action, and 'must' implies necessity, none of which appropriately describe a past ability.