The correct answer is: C. in
The preposition “in” is used to indicate a location inside a building or other structure. In this case, the office is located inside the building on the second floor.
The other prepositions are not as accurate in this context. The preposition “at” is used to indicate a specific location, such as a street address or a point on a map. In this case, the office is not located at a specific point on the second floor, but rather it is located somewhere within the second floor. The preposition “on” is used to indicate a location on the surface of something, such as a table or a wall. In this case, the office is not located on the surface of the second floor, but rather it is located within the second floor. The preposition “of” is used to indicate a part of something, such as a piece of furniture or a part of a body. In this case, the office is not a part of the second floor, but rather it is located within the second floor.