The best and most correct answer among the choices provided by the question is the fourth choice. The phenomenon that Michael Faraday explored was electricity and magnetism. I hope my answer has come to your help. God bless and have a nice day ahead!
Answer:
Part a)

Part b)

Explanation:
Part a)
Electric potential is a scalar quantity
so here we can say that total potential due to a ring on its center is given as

here we know that



now we have


Part b)
Potential on the axis of the ring is given as


