The correct answer is B. nc -v db.comptia.org 3306 because the nc (netcat) command is commonly used to test connectivity to a specific host and port. In this scenario, the administrator needs to verify whether the database service (commonly running on port 3306 for MySQL) is reachable from the system. The -v (verbose) flag provides detailed output about the connection attempt, including whether the connection was successful or refused.

Using nc in this way allows administrators to quickly determine if the issue is related to network connectivity, firewall restrictions, or the service not listening on the expected port. If the connection succeeds, it confirms that the port is open and reachable. If it fails, further troubleshooting can focus on firewall rules, routing, or service availability.

Option A (dig MX db.comptia.org:3306) is incorrect because dig is used for DNS queries, specifically to retrieve DNS records such as MX (mail exchange) records. It does not test port connectivity.

Option C (arp -an | grep db.comptia.org | grep 3306) is incorrect because arp displays the ARP table (IP-to-MAC address mappings) and does not provide information about TCP/UDP port connectivity.

Option D (ss -plant | grep 3306) is incorrect because ss displays local socket statistics and listening ports on the current system. It does not test connectivity to a remote host.

From a Linux+ troubleshooting perspective, tools like nc, telnet, and curl are essential for validating service availability and diagnosing connectivity issues. Netcat is particularly versatile and widely used for quick port checks in network troubleshooting workflows.