Identify the function of the phrase contained in the following sentence.
many scientists have been researching the possibility of black holes.
subject
direct object
predicate noun
adverb
adjective
object of a preposition

I'm not sure what the underlined phrase is, but if it is "the possibility of black holes", then the answer is "direct object". "Many scientists" is the subject, there is no "predicate noun" because there is no linking verb here, there are no adverbs in this sentence, nor adjectives, and the object of a preposition would be "black holes", because it follows a preposition OF.