Answer:
D. Simon and Hillary have been friends since preschool
Step-by-step explanation:
The explicit idea stated in this sentence is: "Simon and Hillary have been friends since preschool"
From the given sentence, it is explicitly stated that Simon and Hillary have been friends since preschool. The "He" in the sentence is actually referring to Simon.
When something is explicit, it means that it is clear enough without any hidden detail. Therefore, the sentence made it clear that Simon and Hillary have been friends since preschool.