The most appropriate antonym of the word ACQUISITION in the given sentence is LOSS.

• Acquisition means the act of gaining or obtaining something.
• Loss means the act of losing or ceasing to have something.

In the given sentence, the players were demoralized after their first loss. This means that they had lost confidence and hope. The coach was surprised by the level of enthusiasm shown by the players, as this was a positive change from their previous state of demoralization. Therefore, the antonym of acquisition in this context is loss.

The other options are not suitable antonyms of acquisition in this context.

• Enthusiasm is a positive emotion, while acquisition is a neutral term.
• Demoralized is a negative emotion, but it does not refer to the act of losing something.
• Surprised is a reaction to something unexpected, but it does not refer to the act of losing something.

Therefore, the correct answer is loss.