According to the people of Mongolia Genghis Khan was one of the cruele
Right Answer is: C
Option C has the grammatically incorrect part. Has must be replaced by have. Relative pronouns like that, who, which etc when used to connect the subject with a verb then the verb must be according to the subject that is antecedent of the relative pronoun. So the antecedent of the relative pronoun here is men which is plural so the helping verb must also be plural. Hence option C is the correct answer.