The correct option is D Down
Let’s examine the sentence: “Please ask your son to turn off the radio so that it is not quite so loud.” The sentence intends to convey the sense of reducing the volume of the radio “so that it is not quite so loud.” The sentence uses the wrong phrasal verb to convey this meaning. The phrasal verb “turn off” means switching off the radio. To convey the sense of reducing the volume of the radio, the phrasal verb “turn down” would be used in the sentence. So, the “off” would be replaced by “down” to correct the sentence. Hence, Option C is the correct answer.