Which step is ineffective in restoring a mobile device's functionality post-security incident?

In the context of restoring a mobile device's functionality after a security incident, opting against the use of multi-factor authentication mechanisms is ineffective. Multi-factor authentication (MFA) is a crucial security measure that enhances account protection by requiring multiple forms of verification before granting access. By not employing MFA, an individual increases the risk of unauthorized access and breaches when attempting to restore the device's functionality.

In contrast, the other options are effective strategies following a security incident. Reinstalling the operating system can eliminate malware or compromised files, ensuring that the device is operating with a clean slate. Resetting network settings helps to remove stuck configurations that might cause connectivity issues post-incident. Additionally, restoring from a clean backup ensures that the device retrieves only safe and secure data, minimizing the risk of bringing back compromised elements. Therefore, the absence of multi-factor authentication undermines the overall security posture and makes the restoration process less effective.