C- 'should have studied' (to express regret) is the ccorrect answer. 'must have studied': we use it to express deduction or conclusion: 'There's nobody at home. They must have gone.' 'might study': we use it to say that something is a possibility: 'It might be true' (= perhaps it is true). 'would study': we use would + infinitive when we imagine a situation or action (= we think of something that is not real): 'It would be nice to buy a new car, but I can't afford it.' (now)