Declarer can count 5 sure winners : A♠, K♠, A♥, A♦ and K♦. He therefore must develop four more tricks to make his 3NT contract.
These additional tricks can come from the ♣ suit in which he holds all high cards except the A♣.
When you have to lose a trick in order to make your contract : lose the trick early, while you still have high card control in the other suits.
Therefore Declarer (at trick 2) leads his Q♣ (high card from the short side first !). The A♣ is not played and Declarer wins the trick.   What does Declarer do next ?