Which type of malicious software "calls home" for further instructions?

The type of malicious software that "calls home" for further instructions is a botnet. Botnets are networks of compromised computers or devices that are infected with malware, allowing an attacker to remotely control them. Once a device becomes part of a botnet, it can receive commands and updates from the attacker's server, which is often referred to as the "command-and-control" (C&C) server. This communication allows the botnet to perform various malicious activities, such as launching Distributed Denial of Service (DDoS) attacks, sending spam, or stealing data.

In contrast, viruses, rootkits, and worms operate differently. A virus typically attaches itself to a legitimate program and replicates itself when that program is run, but does not usually have the capability to "call home." A rootkit is a set of tools designed to gain unauthorized access to a computer while remaining hidden, primarily focused on maintaining access rather than calling for instructions. Worms, while capable of self-replicating and spreading across networks, do not generally communicate with a central command server for further instructions as botnets do. Thus, the defining characteristic of a botnet is its ability to remotely receive commands, making it the correct answer.