'Adverb' is a word that is used to change or qualify the meaning of an adjective, a verb, a clause or another adverb.
In the given sentence, the adjective is 'easy' and 'surprise' is the modification of the adjective.
Option C - 'surprisingly' is an adverb which describes the adjective 'easy'.
On the other hand, Option A - 'surprise' is an abstract noun.
Option B - 'surprised' is verb form in its past tense, which can not modify the verb.
Therefore, the suitable option to complete the given sentence is : Option C - 'surprisingly'.
The completed sentence, thus, becomes : The exam was surprisingly easy.