A window appears on a user's screen staling the computer's files have been encrypted and an access code must be used to unlock the user's data. This is an example of:

Ransomware is a category of malware that encrypts a victim's files and presents a demand requiring payment or a code to obtain the decryption key. The scenario - encrypted files and a required access code to regain data - matches this definition precisely and distinguishes it from other malware types that do not encrypt data or issue ransom demands.

Incorrect file permissions would prevent access to files but would not encrypt their contents or display a ransom-style demand message.

A virus is a broad malware category that self-replicates by attaching to legitimate files; it does not inherently encrypt data and issue payment demands.

An invalid certificate is a PKI or SSL/TLS error that causes browser warnings about a website's identity and has no connection to local file encryption.