When people predict that certain result will not take place unless a

Let's try to simplify the abstract discussion with an example:

"When people predict that certain result will not take place unless a certain action is taken,"

Prediction: You will not get 700+ in GMAT (certain result) unless you study hard (certain action)

"they believe that they have learned that the prediction is correct when the action is taken and the result occurs."

You study hard. You get 700+ in GMAT.
You learn that the prediction is correct.

"On reflection, however, it often becomes clear that the result admits of more than one interpretation."

700+ score admits of more than one interpretation. You may not be able to say that the prediction is correct. You may not be able to say that hard work got you 700+ score.

We need to strength this conclusion.

(A) Judging the success of an action requires specifying the goal of the action.

The goal of the action would presumably be "the result". The conclusion says that there is a specific result. Doesn't help in strengthening the conclusion.

(B) Judging which action to take after a prediction is made requires knowing about other actions that have been successful in similar past situations.

Past situations are irrelevant to the argument. A prediction is made on an action leading to result. The action is already specified. How we arrived at "study hard" is irrelevant here.

(C) Learning whether a certain predictive strategy is good requires knowing the result using that strategy through several trials.

This says that several trials are required to find whether the prediction is true. Our conclusion says that the prediction may not be true at all. Hence this option doesn't strengthen the conclusion either.

(D) Distinguishing a correct prediction and effective action from an incorrect prediction and ineffective action is often impossible.

(D) says that we may not be able to say what "hard work" is. Whether someone worked hard or not cannot be distinguished. If that is the case, then it is true that 700+ score admits of more than one interpretation.

(E) Making a successful prediction requires knowing the facts about the context of that prediction.

Context of the prediction is out of scope.

Answer (D)