Final answer:
The common command-and-control mechanism for botnets is HTTP.
Step-by-step explanation:
A common command-and-control mechanism for botnets is HTTP (Hypertext Transfer Protocol). Botnets are networks of compromised computers that are controlled by a central command server. The command server communicates with the infected computers using various protocols, and HTTP is one of the commonly used protocols for this purpose.
In a botnet, the infected computers, also known as bots, establish a connection to the command server through HTTP. The command server sends instructions to the bots using HTTP requests, and the bots respond with the requested information. This allows the command server to control the bots and carry out malicious activities.
Therefore, the correct answer to the question is HTTP.