Which of the following is the best description of an attack that attaches some malware to a legitimate program so that when the user installs the legitimate program, they inadvertently install the malware?

The best description of an attack that attaches some malware to a legitimate program so that when the user installs the legitimate program, they inadvertently install the malware is a Trojan horse.

A Trojan horse is a type of malware that appears to be a legitimate program but actually contains malicious code. It is often disguised as a harmless or useful program, such as a game or utility, to trick users into installing it. Once installed, the Trojan can perform various malicious activities, such as stealing sensitive information, modifying or deleting files, or giving an attacker unauthorized access to the victim's system.

In the scenario you described, the legitimate program is the "trojanized" or infected program, and the malware is the payload that is installed along with it. The user is unaware of the presence of the malware in the program and unwittingly installs it, which is why this type of attack is also known as a "drive-by download."