Step 1: Analyze the structure of the sentence.
The sentence begins with "Had I learnt acting as a child," which indicates a hypothetical situation in the past. The phrase is a third conditional sentence, which is used to express unreal past situations and their possible outcomes.
Step 2: Understand the choices.
(A) will be: This option suggests a future possibility, but the sentence is about a past unreal condition, so it is incorrect.
(B) can be: This implies a present or future possibility, which does not fit the unreal past condition.
(C) am going to be: This suggests a future intention, which doesn’t fit the context of an unreal past condition.
(D) could have been: This is the correct choice, as it expresses a hypothetical outcome in the past, matching the structure of the third conditional.
Step 3: Conclude.
Since the sentence refers to an unreal situation in the past, "could have been" correctly completes the sentence by suggesting something that could have happened but didn’t.