Correct : a

• The first part of the sentence, "He could not understand the judges awarding her the first prize," indicates a sense of confusion or disagreement with the judges' decision.
• The word "because" introduces the reason for this lack of understanding.
• Therefore, the missing word must describe a performance that is subpar or not good enough to win the first prize.

Evaluate Each Option:
• A) superb: "Superb" means excellent or outstanding. If her performance was superb, he would understand or agree with her getting the first prize. This contradicts the sentence.
• B) medium: "Medium" means average. While an average performance might not win first prize, it doesn't carry the strong negative connotation that would cause someone to "not understand" the decision in such a definitive way.
• C) mediocre: "Mediocre" means of only moderate quality; not very good. This word strongly implies a performance that is distinctly unworthy of a top award like the first prize. This fits perfectly with the sentiment of not understanding why she won.
• D) exhilarating: "Exhilarating" means exciting or thrilling. An exhilarating performance would be a positive attribute and would be a reason *for* her to win, not a reason for him to *not understand* her winning. This contradicts the sentence.

Conclusion:
The word that best expresses a performance unworthy of a first prize, thereby explaining his inability to understand the judges' decision, is "mediocre."