Enabling a user account lockout policy is a security measure that can effectively mitigate brute-force attacks.
After a predetermined number of consecutive failed login attempts, the account will be locked, preventing the attacker from continuing to try different password combinations. This control directly addresses the issue of multiple failed attempts from the same IP address using a single user account, making it the most effective among the options provided. Option B suggests replacing RDP with another remote access tool, which does not address the brute-force attempt but rather avoids the RDP protocol. Option C, implementing a firewall block, could be effective but does not prevent attacks from other IP addresses and may not be as immediate.
Option D, increasing log verbosity, enhances monitoring but does not prevent the attack itself.