Which type of virus or destructive software could appear to be useful, but will actually cause damage once installed or run by a user?

The correct answer is Trojan Horse (Trojan)

A computer virus is a type of computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code into those programs. 

Key Points

Trojan Horse (Trojan)

• It is a type of malware that disguises itself as legitimate code or software.
• Once inside the network, attackers are able to carry out any action that a legitimate user could perform, such as exporting files, modifying data, deleting files or otherwise altering the contents of the device.

Additional Information

Hoax:

• A computer virus hoax is a message warning.
• The recipients of a non-existent computer virus threat.
• The message is usually a chain e-mail that tells the recipients to forward it to everyone they know, but it can also be in the form of a pop-up window.

Worm:

• A computer worm is a subset of the Trojan horse malware that can propagate or self-replicate from one computer to another without human activation after breaching a system.
• Typically, a worm spreads across a network through your Internet or LAN (Local Area Network) connection.

Sector Virus:

• A boot sector is a reserved section of a disk that contains the code and data needed to start the operating system (OS) of a computer.
• A boot sector virus is a type of malware that infects a system's boot partition or the Master Boot Record (MBR) of a hard disk.