The correct option is A (a)
Let’s examine the sentence: “Ten years ago he was having an income, of over ten thousand rupees a month; he must indeed be a wealthy man by now.” The sentence talks about an event that happens ten years ago and it was completed at that time. So, the sentence needs to be framed in past perfect tense using verbs of the same. 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. The sentence is using past continuous tense in its present form and this is not needed. So, the verbs of present continuous tense, i.e. “was having,” needs to be replaced with verbs of past perfect tense, i.e. “had had.” Here, first “had” indicates past tense form of “have” and replaces “was” in the present sentence. The second “had” is the past participle form of “having” and replaces the same in the sentence. So, the error is in Part a of the sentence. Hence, Option A is the correct answer.