Regular verbs are those verbs that change form in the past tense. With most regular verbs, the past simple is created simply by adding -ed. In the given sentence the verb is "perplex". The past simple form of "perplex" is formed by adding -ed to the end of the verb (perplexed). Thus option B is correct while A and C are incorrect. Moreover, the -d ending is usually added to verbs that end with "e".