Concept:The correct phrase is the fixed expression “in bed,” which means being under the bedclothes and ready to sleep.Explanation:“In bed” is the standard idiomatic way to say that someone is lying in bed, usually covered and ready to sleep.“On the bed” means resting on the surface of the bed, not under covers, and it usually requires a determiner like “the” or “my.”Here, the sentence requires the natural fixed phrase “in bed.”Therefore, the best option is C. in.Answer:C. in