The correct answer is: D. No error
The sentence is grammatically correct. The phrase “who lived in the latter half of the nineteenth century” is a relative clause that modifies the noun phrase “transcendental American poets and philosophers.” The relative clause is essential to the
meaning of the sentence, as it identifies the specific group of poets and philosophers that the sentence is about. The sentence is also clear and concise, and it flows smoothly from one clause to the next.