Final answer:
The antecedent of the underlined pronoun 'they' is 'onions'.
Step-by-step explanation:
The antecedent of the underlined pronoun 'they' is 'onions'. In the sentence, 'Onions have antimicrobial properties, and they may also prevent certain cancers', 'they' refers back to 'onions' because onions possess the antimicrobial properties and have the potential to prevent certain cancers. Therefore, the answer is A. onions.