Choose the correct form of the noun to fill the blank in the below sentence:
One of the ___________ in the final exam was tough to solve.

The words "one of" is normally followed by a noun in the plural form. Among the options given below, option B i.e "problems" refers to a plural noun. Hence, this is the correct answer. 

Option A i.e problemes is an incorrect word and carries no meaning. Option C i.e problem, refers to a singular noun. 

Hence both options A and C are incorrect.