You chose that West should play his Ace, and win the trick. This is the correct play(in as much as you have been taught at the moment.)
Since the Ace of Hearts has been played, the rest of the players will just put on their lowest cards to finish off the trick. This means dummy will play the 4 of Hearts, and East will play the 5 of Hearts.
West is now on lead, as he was at the beginning, and he knows quite a bit more about things than he did, when he led the first time. He now knows from seeing the dummy that if he plays his Ace of Diamonds now, he will leave a winning Diamond in dummy, and even though his 10 is a potential winner for later on, there is no way he would ever be able to get back into his hand. Therefore, it is to his advantage not to do that. The only way he will win 2 of the Diamonds, is to lose another one first. He also can make use of the strategy, LOSE YOUR LOSERS FIRST. However, another option is for him to lead into the weakest suit in the dummy which are the Spades. But for the time being, let's concentrate on the chosen Diamond suit.
West leads the 9 of Diamonds, Dummy plays the Queen, East plays the 4 and Declarer plays the 3.
Declarer had no choice but to win that trick, and in doing so, also knows that he has now lost control of that suit. The magic word in No Trump Contracts is CONTROL. You try to get and keep control over all the suits if you can- but it's often not easy to do that against a good defence.
However, Declarer is again in the driving seat, and has done what he had hoped to do with the Hearts. He has established them. They should now provide him with an extra winning trick, unless the distribution in the suit is very unbalanced. So you would then collect your winning hearts. Remember when playing a suit out, that you want to end up being in the hand which has more cards in the suit.
North won the last trick so has to lead to the next one.
19. Which card should be led?
a. King of Hearts
b. Ten of Hearts
c. 7 of Hearts