Concept:The correct adverb before “loud” must express an excessive degree.Explanation:“Too” means more than is desirable or acceptable, and it is used before an adjective like “loud.”“Far” needs “too” or a comparative form; “much” is not used before a positive adjective like “loud”; “enough” comes after an adjective, not before it.Therefore, the sentence should read: “His neighbor complained that the radio was too loud.”Answer:B. too