Which among the following is a malicious software?

A Trojan Horse is a type of malicious software (malware) that disguises itself as a legitimate program but performs harmful actions once installed on a system, such as stealing data, creating backdoors, or damaging files. Hacking and password cracking are activities/techniques (not software themselves). A firewall is a protective security system, not malicious software. Hence, Trojan Horse is the correct answer.