In the sentence "Before the guests began arriving, my mother asked my father if everything was in (its, their) proper place," which pronoun is correct?

In the given sentence, the correct pronoun is "its." This choice is accurate because "everything" is a singular, indefinite pronoun that refers to a collective set of items or arrangements rather than individual items. Therefore, it requires a singular possessive pronoun for proper agreement.

Using "its" indicates that everything being referred to belongs to the entire collection or category of things, meaning they collectively possess a proper place. Conversely, using "their" would imply a plural subject, which does not align with "everything." The other options, "his" and "her," are specific to individual genders and would not apply correctly in this context where the subject is non-specific and singular. Hence, "its" effectively conveys the intended meaning of singular possession in relation to the arrangement of items.