The CCST Cybersecurity course describes ransomware as a form of malicious software that encrypts or locks access to an organization's data, demanding payment for its release.
"Ransomware is a type of malware that denies access to data by encrypting it and demands payment from the victim to restore access. Threat actors may deliver ransomware through phishing emails, malicious downloads, or exploiting vulnerabilities in exposed systems." (CCST Cybersecurity, Essential Security Principles, Malware Types and Threats section, Cisco Networking Academy) A describes spyware or information-stealing malware.
B is website defacement, which is vandalism, not ransomware.
C is correct: locking/encrypting data and demanding payment is the defining behavior of ransomware.
D is more aligned with insider threat or espionage activities.