Select the INCORRECTLY spelt word.

Aquaintance

The incorrectly spelled word is "Aquaintance." The correct spelling is "Acquaintance."

Explanation of all options:

1. Aquaintance: Incorrectly spelled. The correct spelling is "Acquaintance." It refers to a person known to one but not a close friend.

2. Treacherous: Correctly spelled. It means dangerous.

3. Serendipity: Correctly spelled. It refers to the occurrence of pleasant or valuable discoveries by chance.

4. Malicious: Correctly spelled. It describes having the intention to harm or cause trouble to others.