The correct answer is:
Clothing
Explanation:
Modifiers are word or phrases that describe, change, limit, clarifies or qualifies a particular word to add meaning, detail or explanation in a sentence. Modifiers are usually adjectives, adverbs, phrases and adjective or adverb clauses.
In this sentence, the phrase "worn by celebrities" is acting as an adjective clause to the subject which is "clothing". This phrase is a modifier because it specifies that clothing worn by celebrities (not all kinds of clothing, just this one) can be sold at auction for crazy high prices.
Without the modifier phrase, it would be "Clothing can be sold at auction for crazy high prices, and the entire meaning of the sentence would be changed.