What is WRONG in formulation of average inventory in an inventory system with uncertainty?

a$.$

Average Inventory $=$ Cycle Inventory $+$ Safety Inventory

b$.$

Average Inventory $=0\mathrm{.}5*$ Order Quantity $+$ Z$\_$Alpha $*$ Sigma$\_$L

c$.$

Average Inventory $=$ Cycle Inventory $+$ Z$\_$Alpha $*$ Sigma$\_$L

d$.$

Average Inventory $=$ Cycle Inventory $+$ Z$\_$Alpha $*$ Lead Time