What is the formula for the electric field a distance z directly above the center of the ring?

A) \ \1}4\\pi\\epsilon_0} \2\\pi a²\\sigma z}z²+a²³/2}} \
B) \ \1}4\\pi\\epsilon_0} \2\\pi b²\\sigma z}z²+b²³/2}} \
C) \ \1}4\\pi\\epsilon_0} \2\\pi b²-a²\\sigma z}z²+b²-a²)³/2}} \
D) \ \1}4\\pi\\epsilon_0} \2\\pi b²+a²\\sigma z}z²+b²+a²)³/2}} \