Which option is selected to allow only secure dynamic updates in a DNS zone?

The selected option to allow only secure dynamic updates in a DNS zone is the one that specifies this particular security requirement. Secure dynamic updates are a mechanism that enables DNS clients to register and update their DNS records securely, ensuring that only authorized clients can make changes to the DNS information.

By choosing to allow secure dynamic updates only, a DNS zone ensures that updates can only be performed by authenticated and authorized users or systems. This significantly enhances the security of the DNS infrastructure by preventing unauthorized or potentially malicious updates, which could lead to DNS spoofing or other vulnerabilities.

The other options represent varying levels of openness regarding who can update the DNS records. Allowing all dynamic updates would not enforce any security measures, while allowing none would prevent any automated updates, likely hindering administrative tasks. Allowing both secure and insecure updates introduces risks since unauthorized clients could make changes, potentially impacting the integrity of the DNS records. Thus, the correct choice is foundational for maintaining a secure and reliable DNS environment.