Concept:This sentence uses a third conditional.
Third conditional describes an unreal past situation.
Its structure is: if + past perfect, would have + past participle.
Explanation:The main clause is "I would have punished him".
This contains the "would have" part of the third conditional.
So the if-clause must use the past perfect tense.
The past perfect is formed with "had" plus the past participle of the verb.
For the verb "hear", the past participle is "heard".
Thus, the correct phrase is "if I had heard what actually happened".
Option A gives "had heard", which matches this structure correctly.
Options B, C, and D do not form the correct past perfect pattern.
Answer:A. had heard