He is hopeful . . . . . . . . his success in the I.A.S. examination.

The preposition "of" can be used in many different contexts. It can be used to help quantify a time or measurement (e.g. "the fifth of September" or "three pounds of potatoes") and even identify a location (e.g. "south of California"), but it can also create more general relationships between objects and their nouns.