A headless installation is an installation method that does not require a graphical user interface (GUI) or a monitor, keyboard, and mouse. It can be done remotely through a network connection or a command-line interface. A headless installation is suitable for a server that has limited resources and does not need a GUI.
References:
CompTIA Server+ Certification Exam Objectives1, page 14
Server Management: Server Hardware Installation and Management2, Module 2, Lesson 5

Fault tolerance is the ability of a system to continue functioning in the event of a failure of one or more of its components. Fault tolerance can ensure a secondary network path is available if the primary connection fails.
Fault tolerance can be achieved by using redundant components, such as network cards, cables, switches, routers, etc., that can take over the function of the failed component without interrupting the service.
References: https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam- objectives (Objective 2.2)

A RAID controller battery is used to protect the data in the cache memory of the controller in case of a power failure. The cache memory allows the controller to improve the write performance by buffering the data and writing it to the disk in an optimized way. However, if the battery fails, the controller will switch to write-through mode, which means it will write the data directly to the disk without caching. This will reduce the write performance and increase the latency of the system

The technician should install the heaviest server at the bottom of the rack to load the rack properly. Installing the heaviest server at the bottom of the rack helps to balance the weight distribution and prevent the rack from tipping over or collapsing. Installing the heaviest server at the bottom of the rack also makes it easier to access and service the server without lifting or moving it. Installing the heaviest server at any other position in the rack could create instability and safety hazards.

The sc command is used to create, delete, start, stop, pause, or query services on a Windows OS. It can also be used to register a new service by using the create option. References: https://docs.microsoft.com/en-us
/windows-server/administration/windows-commands/sc-create