Which pronoun best completes the sentence: "To commit themselves to ______ new government, the citizens had to sign the National Pact?"

The correct pronoun to complete the sentence is "their." In this context, "their" is a possessive pronoun that indicates ownership or relation to the citizens mentioned in the sentence. The phrase "commit themselves to their new government" means that the citizens are committing to a new government that belongs to or is associated with them.

Using "their" appropriately reflects that the citizens are making a commitment that relates to something that is theirs, which is crucial for conveying both meaning and clarity in the sentence. The other pronouns do not fit this context: "there" indicates a place or location, "they're" is a contraction for "they are," and "them" is an object pronoun. None of these would convey the intended meaning of possession or relationship that "their" provides.