A DHCP reservation is a way to assign a specific IP address to a device based on its MAC address, which is a unique identifier for each network interface card (NIC). When the administrator replaced the NIC, the MAC address of the server changed, and the DHCP server no longer recognized it as the same device. Therefore, the DHCP server assigned a different IP address to the server, which was incorrect for the application. To fix this problem, the administrator needs to modify the DHCP reservation to use the new MAC address of the NIC, so that the server can get the proper IP address.
A WWNN (World Wide Node Name) is a unique identifier for a Fibre Channel node, which is a device that can communicate over a Fibre Channel network. A WWNN is not related to DHCP or IP addresses, and it is not used for DHCP reservations. Therefore, options B and D are incorrect.
Updating the local hosts file with the correct IP address (option C) is also incorrect, because it does not solve the problem of getting the correct IP address from the DHCP server. The hosts file is a local file that maps hostnames to IP addresses, and it is used to override DNS queries. However, it does not affect how the DHCP server assigns IP addresses to devices. Moreover, updating the hosts file manually on every device that needs to communicate with the server is not a scalable or efficient solution.
Reference:
How to reserve IP Address in DHCP Server - Ask Ubuntu
Static IP vs DHCP Reservation - The Tech Journal
How to Configure DHCP Server Reservation in Windows ... - ITIngredients

Explanation
Load balancing is a concept that distributes the workload across multiple servers or other resources to optimize performance, availability, and scalability. Load balancing can be implemented at different layers of the network, such as the application layer, the transport layer, or the network layer. Load balancing can use various algorithms or methods to determine how to distribute the traffic, such as round robin, least connections, or weighted distribution.
References: CompTIA Server+ Study Guide, Chapter 6: Networking, page 241.

Explanation
Link aggregation is the best option to implement on the new server to ensure the least amount of network downtime but a good amount of network speed. Link aggregation is a technique of combining multiple physical network interfaces into one logical interface to increase bandwidth, redundancy, and load balancing.
Link aggregation can improve the performance and availability of the server by allowing it to use more than one network path for data transmission and failover in case of link failure. Link aggregation can be implemented using various protocols, such as IEEE 802.3ad (LACP), Cisco EtherChannel, or Linux bonding. References: [CompTIA Server+ Certification Exam Objectives], Domain 4.0: Networking, Objective
4.1: Given a scenario, configure network settings for servers.