Sam hasn’t been able to play football since he ___ his leg.a) had brokenb) has brokenc) breakd) broke

The correct answer is d) broke.

The sentence is talking about an action that happened at a specific time in the past (when Sam injured his leg) and has an effect on the present (he can't play football now). In English, we use the simple past tense to talk about finished actions in the past, so "broke" is the correct form of the verb to use here.

The other options are incorrect because: a) "had broken" is the past perfect tense, which we use to talk about an action that happened before another action in the past. There's no second past action in this sentence, so this form is incorrect. b) "has broken" is the present perfect tense, which we use to talk about an action that happened at an unspecified time before now. The sentence is talking about a specific time in the past, so this form is incorrect. c) "break" is the base form of the verb, which we don't use to talk about past actions.