Final answer:
The correct completion of the sentence from the student's question is 'He's over the moon.'
Step-by-step explanation:
The sentence should be completed as follows: He’s over the moon because he’s just got A-grades in all his exams. The phrase ‘over the moon’ is an idiom that means extremely happy or delighted.
It makes perfect sense in the context provided, where the student is very pleased with his academic achievements. The idiomatic expression paints a vivid picture of someone being so elated that they feel as high as the moon.
This hyperbolic statement emphasizes the extent of the student's joy. Such expressions add color and emotion to the language, making it more engaging and relatable.