Concept:The correct phrase is beside oneself with an emotion, meaning to be so overcome that one is almost out of control.Explanation:The sentence needs an idiom that describes the parents becoming overwhelmed with worry.The expression beside themselves with worry means almost frantic or unable to stay calm because of anxiety.Options A, B, and C do not form this standard idiomatic phrase with themselves.So the completed sentence is: I didn't tell my parents about the trip because they would be beside themselves with worry.Answer:D. beside