The correct option is C Dihydrogen
Nascent hydrogen is the reactive form of individual hydrogen atom generated in situ in the reaction mixture. They are more reaction than the normal dihydrogen. Hence it a good reducing agent.
Atomic hydrogen is obtained by dissociation of dihydrogen when molecule by passing through an electric arc struck between two tungstant rods at atmospheric pressure. These are the extremely reactive and act as a powerful reducing agent.
Dihydrogen is the poorest reducing agent among the nascent and atomic hydrogen because it is more stable and less reactive than the others.
Hence, option (c) is the correct answer.