Which of the following is an example of social engineering: Installing antivirus software, Tricking people into revealing their passwords, Using complex passwords, or Running a firewall?

Social engineering is a manipulation technique that exploits human error to gain access to private information or systems. It relies on psychological manipulation rather than technical hacking.

Let's evaluate the options:

• Installing antivirus software: This is a technical security measure to protect against malware.
• Tricking people into revealing their passwords: This is a classic example of social engineering, where an attacker manipulates someone into divulging confidential information.
• Using complex passwords: This is a good security practice, a technical measure to make passwords harder to guess or crack.
• Running a firewall: This is a technical security measure that monitors and controls incoming and outgoing network traffic.

The option that describes manipulating people to bypass security is "Tricking people into revealing their passwords."

The final answer is $\boxed{\text{Tricking people into revealing their passwords}}$.