Final answer:
The TTL field is reduced by routers handling the packet; when it reaches zero, the packet is discarded to prevent network issues such as infinite loops.
Step-by-step explanation:
When the value in the TTL (Time to Live) field of a network packet reaches zero, the correct answer is a) The packet is discarded. The TTL field is a mechanism used to prevent data packets from circulating indefinitely on the network. Each router that forwards the packet decreases the TTL value by one. If the TTL value reaches zero before the packet reaches its destination, the router will discard the packet to prevent it from clogging up the network. This is a safety feature that helps to control the flow of traffic and ensures network stability by removing packets that are caught in routing loops.