The correct option is B b
Let’s examine the sentence: “Its color makes the moth undistinguished from the branch it rests on.” The sentence wants to convey the meaning that the color of the moth matches with the color of the branch and so it is hard to distinguish moth as a separate entity from branch. To convey this meaning, it should use the adjective “indistinguishable” instead of “undistinguished.” The adjective “undistinguished” means normal and plain and not with any unique feature. So, the error is in Part b of the sentence. Hence, option B is the correct answer.