Which of the following attacks is spread by attaching themselves to files?

Worms can spread by attaching themselves to files and propagating when those files are shared or opened, allowing them to replicate across networks and systems. This file-attachment propagation mechanism distinguishes them from other malware categories in the context of this question.

A botnet is a network of compromised machines controlled by an attacker - it is an infrastructure for launching attacks, not a self-propagating malware type that attaches to files.

A DDoS (Distributed Denial of Service) attack floods a target with traffic to make it unavailable - it is an attack method, not a type of malware that spreads via file attachment.

An FTP bounce attack exploits the FTP PORT command to scan or attack third-party systems - it is a network attack technique, not malware that spreads by attaching to files.