The terrible housing conditions prompted Richard to become politically involved. By the end of the year, he to organize a grassroots campaign in the neighborhood

had begun

Explanation:

The simple past tense and past participle forms of the verb begin and begin, respectively.                                              Whenever we talk about the past perfect tense, we use the past participle form of the verb.                                                       So, correct answer is had begun

It’s because it’s was very fun