What is the major difference between a worm and a Trojan horse?

Email propagation is not a defining or exclusive characteristic of worms - they also spread via network shares, unpatched vulnerabilities, and removable media - so email is not the major technical differentiator.

A Trojan horse is also a form of malicious program, not a utility; classifying it as a utility is technically incorrect since both worms and Trojans are categorized as malware.

A worm is specifically defined by its self-replication capability - it copies itself and spreads across networks without user interaction. A Trojan horse, by contrast, does not replicate itself and must be deliberately installed or run by a user who has been deceived into believing the program is legitimate software. This self-replication capability is the primary technical differentiator between the two malware families.

This answer is entirely reversed - a worm is malicious software, not antivirus software, and a Trojan horse is also malicious, making both parts of this choice factually wrong.