The best metric for a network administrator to check is hop-by-hop network latency. This is because network latency is the time it takes for a packet to travel from the source to the destination, and it affects the loading speed of web pages. Hop-by-hop network latency measures the latency between each pair of routers or switches along the network path, and it can help identify where the delay is occurring. By checking the hop-by-hop network latency, the network administrator can determine if the problem is caused by a slow or congested link, a misconfigured or faulty device, or a routing issue.
Jitter is the variation in latency over time, and it affects the quality of voice and video applications. Jitter does not directly affect the loading speed of web pages, and it is not a useful metric for troubleshooting this issue.
Server interface CRC errors are errors that occur when the cyclic redundancy check (CRC) of a packet does not match the expected value, indicating data corruption. Server interface CRC errors can affect the reliability and integrity of data transmission, and they can be caused by faulty cables, connectors, or interfaces. Server interface CRC errors do not necessarily affect the loading speed of web pages, unless they are severe enough to cause retransmissions or packet loss.
Server NetFlow data is data that is collected and analyzed by the NetFlow protocol, which monitors and reports on network traffic flows. Server NetFlow data can provide information on the volume, type, and direction of traffic that is sent or received by the server, as well as the source and destination IP addresses, ports, and protocols. Server NetFlow data can help identify network usage patterns, trends, and anomalies, but it does not measure the latency or performance of the network.
Reference
What is Network Latency and How to Measure It
How to Troubleshoot Network Latency Issues
What is Jitter and How to Measure It
What is CRC Error and How to Fix It
What is NetFlow and How Does It Work
CompTIA Network+ Certification All-in-One Exam Guide, Eighth Edition (Exam N10-008)

An email protection gateway is an appliance that can filter and block malicious emails and attachments before they reach the recipients. An email protection gateway can mitigate the risk of hyperlinks from inbound emails by scanning the links for malicious content, rewriting the links to point to a safe domain, or blocking the links altogether. An email protection gateway can also perform other functions such as spam filtering, antivirus scanning, encryption, and data loss prevention. A DNS server, a proxy server, an endpoint email client, and a sandbox are not appliances that can enable this capability, as they have different purposes and functions.
References
* 1: CompTIA Network+ N10-008 Certification Study Guide, page 304
* 2: CompTIA Network+ N10-008 Exam Subnetting Quiz, question 15
* 3: CompTIA Network+ N10-008 Certification Practice Test, question 5
* 4: Email Protection Gateway - N10-008 CompTIA Network+ : 3.2

A toner is a tool that generates an audible signal that can be traced by a probe. A network technician can use a toner to identify the appropriate patch panel port by connecting the toner to one end of the patch cord and using the probe to scan the patch panel until the signal is detected. A toner is the easiest way to identify the patch panel port when the patch panel is not labeled, as it does not require a laptop, a cable tester, or a visual fault locator. A toner can also be used to locate breaks or shorts in a cable, or to verify continuity.
Reference:
Using a Toner and Probe - CompTIA Network+ Certification (N10-008): The Total Course Video CompTIA Network+ Certification Exam Objectives, page 141

Explanation
Client roaming refers to the ability of a wireless device to seamlessly connect to a different access point (AP) as the user moves around the facility. This can help to improve network stability and reduce interruptions in mobile applications. An extended service deployment is a type of wireless network configuration that uses multiple APs to cover a large area, such as a warehouse facility. By using a wireless controller to manage the APs, the network can be better optimized for client roaming, which can improve network stability.
"Roaming With multiple WAPs in an ESS, clients will connect to whichever WAP has the strongest signal. As clients move through the space covered by the broadcast area, they will change WAP connections seamlessly, a process called roaming."

DHCP snooping is used to prevent unauthorized DHCP servers from assigning IP addresses and other configuration settings, such as DNS servers, to network hosts. By implementing DHCP snooping, you can protect against attacks where a rogue DHCP server provides malicious configuration information, such as a fake DNS server.