Concept:The phrase “what was eating him” is an idiom that means what was troubling or worrying him.Explanation:The idiom “eating him” does not refer to food or physical health.It is used to describe a feeling that causes mental distress or annoyance.In this context, the mother wants to know the reason for his disturbed mood.Therefore, she is trying to find out why he was upset.Answer:Option C — why he was upset.