The correct answer is D. his.
The members of the religious sect ostracized the recreant who had abandoned his faith. This means that they shunned him and refused to associate with him because he had stopped believing in their religion.
The other options are incorrect because they do not agree with the meaning of the sentence. Option A, they, is plural and refers to the members of the religious sect. Option B, them, is also plural and refers to the members of the religious sect. Option C, their, is a possessive pronoun and refers to the faith of the members of the religious sect.
The sentence is about a single person, the recreant, who has abandoned his faith. Therefore, the correct pronoun to use is his.