Which of the following protects a mobile device against unwanted access when it is left unattended?

A PIN code (Personal Identification Number) is a fundamental security measure for mobile devices that prevents unauthorized access by requiring the user to enter a specific numerical sequence to unlock the device's screen and access its contents when left unattended.

OS updates apply security patches and feature enhancements, which protect against vulnerabilities but do not prevent direct physical access to an unattended, unlocked device.

Antivirus software protects against malicious software infections but does not prevent someone from physically accessing an unattended device if it is unlocked or if the screen lock is bypassed.

A BYOD (Bring Your Own Device) policy is a set of rules for using personal devices in a corporate environment; it is a policy, not a technical control that protects an individual device from direct physical access.