Which type of malware is primarily used to track user activity and steal sensitive information?

The correct answer is spyware. This type of malware is specifically designed to monitor user activity and gather personal information without the user's consent. It can track online activities, keystrokes, and capture sensitive information such as passwords, credit card numbers, and other private data. By operating stealthily in the background, spyware can send the collected data to malicious actors, who exploit it for identity theft, fraud, or other malicious purposes.

In contrast, adware generates advertisements and typically serves promotional content, often without the intent to steal personal data. Rootkits are designed to gain unauthorized access to a system and maintain that access, but they do not inherently focus on tracking user activity or stealing data. Worms are a type of malware that replicates itself to spread to other computers but are not specifically used for tracking or stealing information. Thus, spyware is distinct in its primary function of monitoring and capturing sensitive information from users.