Which of the following strategies will mitigate the risk of a zero-day vulnerability MOST efficiently?

Using open-source technologies does not reduce zero-day exposure, as open-source software is equally susceptible to unknown vulnerabilities and may have fewer dedicated security researchers monitoring it.

Keeping resources up to date patches known, disclosed vulnerabilities - but zero-day vulnerabilities have no available patch yet, so updates cannot address them.

A standby environment with a different cloud provider provides redundancy and disaster recovery, but does not prevent or mitigate the exploitation of a zero-day vulnerability.

A zero-day vulnerability is by definition unknown to the vendor and has no patch available, which makes any patch-based or update-based defense ineffective before disclosure. A detailed incident response plan prepares the organization to quickly detect, isolate, and remediate exploitation as soon as a zero-day is identified, minimizing damage and recovery time. This is the most efficient mitigation strategy because it addresses the reality that the threat cannot be eliminated before it is discovered.