The correct answer is A. Only
Step-by-step explanation
It is known as a modifier to words or phrases that act by modifying the core of the subject in a sentence, complementing its meaning. Among the modifiers of the subject's nucleus, there are the articles and adjectives that accompany the subject's nucleus noun, define it, and agree with it in gender and number. These modifiers are called direct modifiers. Therefore, Only is a modifier because it modifies the complete meaning of the phrase by modifying the core noun of the subject "they". So the correct answer is A. Only.