Which defect in a vehicle causes it to fail inspection before ammunition transport using DD Form 626?

The correct choice indicates a defect in a vehicle that is critical for safety and visibility during ammunition transport. A cracked driver's mirror can significantly impair the driver's ability to see other vehicles or obstacles, which poses a safety risk. Safe navigation is especially crucial when transporting potentially hazardous materials like ammunition.

While other options may represent inconveniences or legal responsibilities (such as outdated registration or insufficient fuel), they do not inherently risk a safe transport environment to the same degree. Malfunctioning headlights, while important for visibility, may still allow for some degree of safe driving during daylight or under certain conditions, but a compromised driver's mirror affects situational awareness directly. Hence, a cracked driver's mirror is a more severe defect warranting failure of inspection prior to ammunition transport.