The correct answer is 'A. Trojan horse' because a Trojan horse is a type of malware that appears to be harmless or even beneficial, but actually contains malicious code that can disrupt a computer system. Unlike viruses or worms, Trojan horses do not replicate themselves but instead rely on users to unknowingly install them. Therefore, a Trojan horse fits the description of an intentionally disruptive program that spreads from program to program or from disk to disk.