Which TTL value causes a router to drop a packet?

The Time To Live (TTL) value is a field in the IP protocol that helps prevent packets from circulating indefinitely in the network. It is essentially a counter that decrements by one each time the packet is processed by a router. When the TTL value reaches zero, the router discards the packet and typically sends an Internet Control Message Protocol (ICMP) message back to the source indicating that the packet could not be delivered.

In this case, a TTL value of zero means that the packet has expired and should be dropped by the router, as it indicates that the packet has traveled as far as it is allowed to based on the time-to-live parameter.

Other TTL values like 1, 64, or 15 would allow the packet to pass through at least one router (or in the case of a TTL of 1, only one router before being decremented to zero). Thus, those values would not cause a router to drop the packet immediately; they still provide some allowance for routing. In summary, a TTL of zero is explicitly designated to trigger the packet's drop by routers, making it the correct choice.