Explanation
Port 23 should be blocked when checking firewall configuration to prevent outside users from telnetting into any of the servers at the datacenter. Port 23 is the default port for Telnet, which is an insecure protocol that allows remote access to servers and network devices. Telnet sends data in clear text, which can be easily intercepted and compromised by attackers. A more secure alternative is SSH, which uses port 22 and encrypts data. References:
https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html

A short circuit is a condition where two conductors in a circuit are connected unintentionally, creating a low resistance path for the current. This causes the voltage to drop and the current to increase, which can damage the circuit or cause a fire. A multimeter can measure the resistance or impedance of a circuit, and if it shows extremely low values, it indicates a short circuit.

A hot site is a backup site that is fully equipped and ready to take over the operations of the primary site in the event of a disaster. A hot site has real-time synchronization with the primary site and can provide zero data loss. A hot site is the most expensive and reliable option for disaster recovery.
References: Network+ Study Guide Objective 5.3: Explain common scanning, monitoring and patching processes and summarize their expected outputs.

Port 5900 is the default port used by Virtual Network Computing (VNC), a graphical desktop-sharing system that allows remote access to another computer's screen, keyboard and mouse1. VNC is one of the tools that can be used for remote troubleshooting of servers with high CPU usage, as it enables the technician to see the server's desktop and perform actions on it. Port 5900 is also one of the ports listed in the CompTIA Network+ N10-008 exam objectives under the section "1.4 Given a scenario, use appropriate software tools to troubleshoot connectivity issues"2.
The other ports are not commonly used for remote access. Port 443 is used for Hypertext Transfer Protocol Secure (HTTPS), a secure version of HTTP that encrypts the communication between a web browser and a web server3. Port 3321 is used for Telepathy Broker Service, a service that provides communication between applications and telephony devices4. Port 8080 is used for HTTP Alternate, an alternative port for HTTP that is often used by web servers for testing or proxy purposes5.
Reference
1: Port 5900: What Is It Used for & How to Open It for RDP
2: Network+ (Plus) Certification | CompTIA IT Certifications
3: [What is HTTPS? - Definition from WhatIs.com]
4: [Telepathy Broker Service - Port 3321 - TCP/UDP]
5: [What is Port 8080? Why do some processes use this port?]