Which of the following is a common technique used by spyware?

Spyware is designed to gather information from a user's device without their knowledge or consent. This typically includes collecting personal data such as browsing habits, login credentials, and sensitive information. The purpose of this unauthorized collection is often to track users for advertising or to steal identity-related information.

Option B directly describes the primary function of spyware, which is to covertly obtain personal information without the user's awareness, making it the most accurate representation of a common technique employed by spyware.

In contrast, other options describe activities that are not characteristic of spyware. Encrypting files is a method used to secure data from unauthorized access, which is not the goal of spyware. Optimizing system performance is typically a function of legitimate software designed to improve the efficiency of a system, rather than a characteristic of spyware. Routing internet traffic is relevant to network management or proxy services, but again, it does not encapsulate the illicit data-gathering behavior that defines spyware.