Question 30: Had you known that the river was dangerous, he ………… to swim across it.

Inversion structure of 3rd conditional sentence Had S + P2 +…, S would have + P2+…

Based on the above structure, only two answers A and D are suitable

Based on the meaning of the sentence => Answer D is the most suitable

If he knew the river was dangerous, he wouldn’t have tried to swim across it.