What is a PDU $($Protocol Data Unit$)?$

a$.$ It is an Application layer protocol that handles delivery of messages.

b$.$ It is a unit of measurement for the maximum amount of data that can be transferred.

c$.$ It is the name given to the unit of information used by each layer of the OSI $($Open Systems Interconnection$)$ model.

d$.$ It is a Network layer protocol that handles path determination.