Concept:In this sentence, defend means to give good reasons or arguments in favour of a decision.Explanation:“This is one decision you cannot defend” means you cannot show that the decision was right or reasonable. The word that best matches this meaning is justify. Explain only means to make something clear, not to prove it right. Support is close, but the key idea here is proving or defending the correctness of the decision. Express does not fit the intended meaning.Answer:A. justify