Concept:The relative pronoun “who” refers to a plural noun, so it must take a plural verb.Explanation:The phrase “one of the ablest persons” means the group of persons is plural.Therefore, “who” stands for “persons” and requires a plural verb: “have written”, not “has written”.Thus part (c) contains the error.The correct sentence is: “This view has been taken by one of the ablest persons who have written on this subject.”Answer:(c)