Which changes in costs are most conducive to switching from a traditional inventory
ordering system to a just-in-time ordering system?

Cost per Purchase Order
Inventory Unit Carrying Costs

Answer (B) is correct.
A JIT system is intended to minimize inventory. Thus, if inventory
carrying costs are increasing, a JIT system becomes more cost effective.
Moreover, purchases are more frequent in a JIT system. Accordingly, a
decreasing cost per purchase order is conducive to switching to a JIT
system.