Select the correct pronoun: "Everyone submitted (his or her, their) research proposals ahead of the deadline."

In the sentence, "Everyone submitted (his or her, their) research proposals ahead of the deadline," the usage of "his or her" is considered more grammatically formal and ensures gender neutrality, as it explicitly acknowledges both genders. It refers to an individual, and the phrase "his or her" correctly matches with "everyone," which is a singular pronoun.

Using "their" as the correct pronoun in casual or modern usage, however, has gained acceptance in many contexts as a way to maintain inclusivity without being overly formal. "Their" serves as a singular they, which is used to avoid gender specificity while still referring to an individual in a collective sense.

The choice of "their" reflects an evolving understanding of language and inclusivity, making it a valid choice for modern English usage, especially in less formal contexts. Thus, it can be seen as an appropriate and correct pronoun to use here, as it connects to contemporary preferences for inclusive language.