Final answer:
Correct option is D) .In messaging systems, the essential component installed by the system administrator is the queuing object in the server, which handles incoming messages and ensures their reliable delivery.
Step-by-step explanation:
The component of a messaging system that must be installed by a system administrator is d. The queuing object in the server. Messaging systems rely on server components to manage the transfer of messages between clients. A queuing mechanism on the server is crucial because it buffers incoming messages, orders them if necessary, and ensures they are reliably delivered to the correct receivers. Installing this component on the server allows the system to decouple the sender and receiver, meaning that the sender can continue operations without waiting for the receiver to acknowledge receipt.