Select the letter of the term that correctly identifies the function of
the word or phrase in parentheses in the following sentence:
most collectors consider fossils (priceless).

The term that correctly identifies the function of the word priceless is object complement. 
Since the word fossils is object, then the word that completes it is object complement.
Although you may be tempted to say it is predicate adjective, it cannot be, because predicate adjectives refer to the subject of the sentence.