Answer and explanation

Correct answer: D. should

  1. Step 1: 'Should have' expresses regret about something not done.
Answer
should.
Why not the others: must, can't and will don't fit.

Practise more Advanced Grammar, Clauses and Connectors.

Free questions with an explanation for every one, and full timed YDS (Yabancı Dil Bilgisi Seviye Tespit Sınavı) mock tests.