The confusion here is about the correct usage of “who” and “whom.”
“Who” is used as a subject, while “whom” is used as an object.
In the clause “who they say is the best singer,” “who” is the subject of “is the best singer,” and “they” is the subject of “say.” Therefore, “who” is correct, not “whom.”
Options (A) and (C) incorrectly use “whom” where “who” is required.
Option (D) has awkward comma placement — “who, they say is” is missing a second comma after “say” to correctly mark the parenthetical phrase “they say.”
Option (B) is correct in both grammar and punctuation: “who they say is the best singer” flows naturally and is grammatically sound.