The correct preposition for the above given sentence is “of” because it form a correctly grammatical sentence i.e. The hospital is located at the top of the hill.
The other words such as “off”, “on” or “in” do not represent a correct sentence formation to denote subject (hospital) location clearly.
Thus, the options (c), (b) and (a) are incorrect.
The correct preposition for the above given sentence is “of” because it form a correctly grammatical sentence i.e. The hospital is located at the top of the hill.
Therefore, the correct option is – a. of.