Concept:Prepositions show the relationship between a person or object and a place.
Explanation:The sentence is: "I found my friends sitting ___ the beach."
When people sit on the sand at a beach, we say they are sitting
on the beach.
The preposition
on is used for a surface that supports someone or something.
In would suggest being enclosed or surrounded, which is not correct for an open beach.
Under means below or covered by something, so it does not fit the meaning.
At only gives a general area; it does not clearly show that the friends were seated on the sandy surface.
Therefore, the best word to complete the sentence is
on: "I found my friends sitting on the beach."
Answer:D. on