Explanation
tracert is a command-line tool that can trace the route of a packet from the source to the destination. It can show the number of hops, the IP address and hostname of each router, and the round-trip time for each hop.
tracert can help the technician troubleshoot the unreachable website by identifying where the packet is dropped or delayed along the path. ipconfig is a command-line tool that can display and configure the IP settings of a network interface. arp is a command-line tool that can display and manipulate the Address Resolution Protocol (ARP) cache, which maps IP addresses to MAC addresses. netstat is a command-line tool that can display network connections, routing tables, and statistics.
References: CompTIA Network+ Certification Exam Objectives Version 7.0 (N10-007), Objective 2.4: Given a scenario, use appropriate software tools to troubleshoot connectivity issues.

A customer needs six usable IP addresses, which means they need a subnet that can accommodate at least six hosts, excluding the network address and the broadcast address12.
The subnet mask determines how many bits are used for the network prefix and how many bits are used for the host identifier12. The more bits are used for the network prefix, the fewer bits are available for the host identifier, and vice versa12.
The number of usable IP addresses in a subnet can be calculated by subtracting two from the number of possible host identifiers, which is 2^(number of host bits)12.
For example, a subnet mask of 255.255.255.0 (/24 in CIDR notation) has 24 bits for the network prefix and 8 bits for the host identifier. Therefore, the number of usable IP addresses in this subnet is 2^8 - 2 = 25412.
The four options in the question have different subnet masks, which correspond to different numbers of usable IP addresses, as shown in the table below:
Table
Subnet mask
CIDR notation
Number of host bits
Number of usable IP addresses
255.255.255.128
/25
7
126
255.255.255.192
/26
6
62
255.255.255.224
/27
5
30
255.255.255.240
/28
4
14
Therefore, the best option that meets the customer's requirement of six usable IP addresses is C. 255.255.255.224, which has 30 usable IP addresses in the subnet12.
Reference:
1: Subnet Cheat Sheet - 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network Reference
2: Network Subnets (CIDR) and Usable IP Addresses Reference Guide

A proxy is a solution that can mediate access to the internet. A proxy is a server that acts as an intermediary between a client and a destination server on the internet. A proxy can perform various functions, such as filtering, caching, logging, or modifying requests and responses. A proxy can help improve network performance, security, and privacy by reducing bandwidth consumption, blocking malicious or unwanted content, monitoring network activity, and hiding the client's identity or location. Reference: [CompTIA Network+ Certification Exam Objectives], What Is a Proxy Server? | Proxy Server Definition | Avast

RADIUS (Remote Authentication Dial-In User Service) is a protocol that should be used by the organization to allow for authentication, authorization, and auditing of network services. RADIUS is an AAA (Authentication, Authorization, and Accounting) protocol that manages network access by verifying user credentials, granting access permissions, and logging user activities. RADIUS uses a client-server model where a RADIUS client (such as a router, switch, or VPN server) sends user information to a RADIUS server (such as an authentication server) for verification and authorization. The RADIUS server can also send accounting information to another server for billing or reporting purposes. Reference: https://www.cisco.com/c/en/us/support/docs/security-vpn/remote-authentication-dial-user-service-radius/13838-10.html

Explanation
A reservation should be configured to ensure a device with a specific MAC address is always assigned the same IP address from DHCP. A reservation is a feature of DHCP that allows an administrator to assign a fixed IP address to a device based on its MAC address. This way, the device will always receive the same IP address from the DHCP server, even if it is powered off or disconnected from the network for a long time. References:
https://docs.microsoft.com/en-us/windows-server/troubleshoot/configure-dhcp-reservations