Final answer:
In a B-train combination, the relay valve with the lowest crack pressure is typically found on the pup axle to ensure responsive braking at the rear of the combination for improved control and stability.
Step-by-step explanation:
In a B-train combination, which is a specific configuration of a tractor-trailer setup typically used in heavy-duty transportation, relay valves are used in the air braking system to apply and release the brakes. The relay valve that would likely have the lowest crack pressure is typically designed to respond quickly to ensure safe braking performance. Crack pressure is the minimum pressure at which a valve begins to pass fluid or air.
The relay valve with the lowest crack pressure is most often found on the pup axle, part c). The reason for this is due to the requirement for the brakes on the rear of the combination (which includes the pup trailer) to apply slightly earlier than those on the tractor or the lead trailer. This helps in maintaining better control and stability of the B-train combination during braking.