Tailgating is a type of physical security attack in which an unauthorized person follows an authorized person into a restricted area, such as a data center, without proper identification or authentication. Tailgating can allow attackers to access sensitive data, equipment, or network resources, or to plant malicious devices or software. The network administrator prevented tailgating by stopping and directing the unknown person to the security desk, where they would have to verify their identity and purpose.
References:Digital Threats and Cyberattacks at the Network LevelNetwork attacks and how to prevent them.

Explanation
The time-to-live (TTL) field is the mechanism for ending loops at Layer 3, which is the network layer of the OSI model. The TTL field is an 8-bit field that indicates the maximum time or number of hops that an IP packet can travel before it is discarded. Every time an IP packet passes through a router, the router decrements the TTL value by one. If the TTL value reaches zero, the router drops the packet and sends an ICMP message back to the source, informing that the packet has expired. This way, the TTL field prevents an IP packet from looping endlessly in a network with routing errors or cycles123.
The other options are not mechanisms for ending loops at Layer 3. The checksum field is a 16-bit field that is used to verify the integrity of the IP header. The checksum field is calculated by adding all the 16-bit words in the header and taking the one's complement of the result. If the checksum field does not match the calculated value, the IP packet is considered corrupted and discarded12. The type field, also known as the type of service (TOS) or differentiated services code point (DSCP) field, is an 8-bit field that is used to specify the quality of service (QoS) or priority of the IP packet. The type field can indicate how the packet should be handled in terms of delay, throughput, reliability, or cost12. The protocol field is an 8-bit field that is used to identify the transport layer protocol that is encapsulated in the IP packet. The protocol field can indicate whether the payload is a TCP segment, a UDP datagram, an ICMP message, or another protocol12.

The best command for the administrator to run first on the switch is show vlan, which displays the VLAN configuration information on the switch, such as the VLAN IDs, names, and ports. The show vlan command can help the administrator to verify if the printer is assigned to the correct VLAN, and if there is any mismatch or misconfiguration that prevents the printer from being reachable. The show route command displays the routing table on the switch, which is not relevant for the printer connectivity, as the printer is configured with the correct gateway. The show arp command displays the ARP cache on the switch, which maps the IP addresses to the MAC addresses of the devices. The show arp command can help the administrator to check if the switch has learned the MAC address of the printer, but it is not the best command to run first, as the ARP cache can be cleared or updated frequently. The show config command displays the running configuration on the switch, which includes the VLAN configuration, but it also includes other information that is not related to the printer issue, such as the hostname, the interface settings, the security settings, and so on. The show config command can be overwhelming and time-consuming to analyze, so it is not the best command to run first.
Reference
1: VLANs and Trunking - N10-008 CompTIA Network+ : 2.3
2: N10-008 CompTIA Network+ : 5.5
3: Interface Configurations - N10-008 CompTIA Network+ : 2.3
4: Configuring VLANs - CompTIA Network+ N10-006 - 2.6

Explanation
Asymmetrical routing is the most likely cause for the WAN instability. When two different routing protocols are used, like OSPF and EIGRP, it can cause asymmetrical routing, which results in traffic being routed differently in each direction. This can lead to instability in the WAN. A CDP neighbor change, a switching loop, or an incorrect IP address are not likely causes for WAN instability.