Final answer:
Spyware is a type of software that monitors and collects information about a user's activities on a computer without their knowledge or consent.
Step-by-step explanation:
Spyware is a type of malicious software that monitors and collects information about a user's activities on a computer without their knowledge or consent. This information can include keystrokes, web browsing history, passwords, and other sensitive data. Spyware is designed to secretly transmit this information to a third party. Therefore, the best definition of spyware is option D: Software that monitors activity on a computer.