To be honest, this is kind of an incorrect way to start thinking about Modifiers. The job of a modifier in a sentence is to describe; any part of a sentence (verb, phrase, clause) that describes any other part of the sentence is a modifier.
In the sentence "She placed the cat on the couch" the phrase "on the couch" does not describe the cat, it does not describe the manner of placing, it doesn't describe anything. In this case, it is simply the object of the sentence.
In fact, most prepositional phrases are simply objects, since they are often noun phrases that are not doing an action in a sentence.
Rather than looking at whether something is answered or not, you wanna instead ask yourself what entity/action is being described here. As it is, the GMAT does not require you different between adjective and adverb modifiers, it only asks you to make sure that the descriptor/modifier is describing/modifying the noun or noun phrase or verb or clause that it logically should describe within the sentence.
- Matoo