$4\mathrm{.}01-4.$ Full buffers! Suppose that the output port has enough buffer space for only three packets, including the packet being transmitted $($in practice, routers have a lot more buffer space!$).$ What happens to packet $4$ in the example above, when it arrives to find the output buffers full?

a$.$ Packet $4$ is held in the switch fabric and released to the output port when there is free buffer space.

b$.$ Packet $4$ is recirculated back to input port and buffered there.

c$.$ Packet $4$ is dropped or a queued packet is dropped from the buffer to make space for packet $4.$