What is the source of this question? Can something like this come on the GRE?

Hello,
why not A?

Not a GRE question

Do not waste your time

Official explanation from Manhattan GRE:

Context for the blank in this sentence is rather tricky. We know that the action “was not a duty,” and we know that it was a “sacrifice.” So, Kelly sacrificed not out of necessity but of her own will. Sometimes the clues in the sentence give us our filler directly – “unnecessary sacrifice” is what we’re looking for. Note that we’re looking for something positive.

(A) Unnecessary certainly works for the context, implying that the action was not required as a duty.

(B) Perfidious, meaning “treacherous or deceitful,” could describe an action that is not a duty, but it does not fit with the clue word “sacrifice.” Perfidious is negative.

(C) Disloyal actions would clearly not be duties, but again disloyal is negative and does not imply “sacrifice.”

(D) CORRECT. Supererogatory describes “an action performed beyond what is expected or required.” This seems to work!

(E) Retrograde means “regressing or moving backwards” and does not make sense in the sentence.

(F) CORRECT. Above and beyond the call is an idiom typically used in military contexts to describe actions that are above and beyond the call of duty or more than duty requires. This is a good fit.

Answer choice (A) fits well in the blank but no other answer choice could fill the blank and give the sentence similar meaning. Answers (B) and (C) if chosen together would give the sentence a similar meaning. However, these words would not imply “sacrifices.”

I was not her duty, yet she still did it. She made a sacrifice. It was beyond the call of her duty. The choices are supererogatory and above and beyond the call .

The word supererogatory means "a good act that is in excess of what is morally or legally required."

Even if you did not know the meaning, you can get the answers by the process of elimination. Since we are looking for something positive, we can eliminate unnecessary, disloyal, perfidious and retrograde. Now we have the correct answers.