The correct option is B b
Let’s examine the sentence: “Old age and infirmity had began to catch up with him.” The sentence is in past perfect tense as indicated by the use of the helping verb “had.” Past perfect tense indicates that an action was completed (finished or "perfected") at some point in the past before something else happened. This tense is formed with the past tense form of "to have," i.e. had, plus the past participle of the verb. This sentence does not use past participle of the verb “begin” and hence, it requires to be corrected here. The sentence should use “begun” instead of “began”. So, the error is in Part b of the sentence. Hence, option B is the correct answer.