Final answer:
The correct pronoun to complete the sentence 'Has either candidate argued ______ side in the debate yet?' is 'their', as it is gender-neutral and matches the singular indefinite nature of 'either candidate'.
Step-by-step explanation:
The sentence in question requires a pronoun that matches the singular and indefinite nature of 'either candidate.' Traditionally, male pronouns were used as a default in such cases, but modern usage embraces gender neutrality. Therefore, the best choice is to use their as the pronoun, which is both singular and non-binary. This aligns with current conventions that aim to be inclusive and avoid gender bias. Hence, the sentence should read: Has either candidate argued their side in the debate yet?