Which document is essential for international travel?

A passport is essential for international travel as it serves as the primary form of identification recognized by governments around the world. It allows travelers to enter foreign countries, providing proof of citizenship and identity, and it is often required for visa applications. Unlike a driver's license, which is valid only within the issuing country, or a birth certificate, which does not serve as an accepted identification for travel purposes, a passport is specifically designed for international travel. Similarly, a social security card does not provide sufficient identification for crossing international borders. Therefore, a passport is the only document among the options listed that is universally accepted and required for international journeys.