His dog sat ___3___ him.
Prepositions are words that indicate relationships with the other words in the sentence.
The given sentence should be filled with a preposition of place since it speaks about the place where the dog sat.
Prepositions 'at', 'on' are not relevant in the given context. As 'on' means on top of, whereas 'in' means within a enclosed space. Therefore options B and C are incorrect.
Whereas 'besides' is not a preposition but an adverb, which cannot be used here, therefore option C is incorrect.
Preposition 'beside' means lay next to, which is apt here. Therefore option D is the correct answer.