The police questioned everyone in the room.
Here "in the room" is an adjective phrase or an adverb phrase
I think it is an adverb phrase.But some of the teachers of our country think it an adjective phrase. Which one will be correct?
This is a textbook case of ambiguous meanings. Two interpretations are possible:
The sentence can correctly be parsed either way. Only context will tell you which interpretation is the intended one.
The police questioned [everyone in the room].
The bracketed element is neither an adverb phrase nor an adjective phrase.
"In" is a preposition, so "in the room" can only belong to the category 'preposition phrase'.
Note that adjective and adverb phrases are ones headed by an adjective / adverb:
It's a [very nice house] (adjective phrase)
I dealt with it [quite separately]" (adverb phrase)
Functionally, the most salient interpretation of your example is that the PP is modifying the noun "everyone".