Which form acts as proof that the provided vehicle meets requirements to transport ammunition?

The correct answer is DD Form 626, which is specifically utilized to certify that a vehicle meets the necessary requirements for transporting ammunition. This form ensures that the vehicle adheres to safety and regulatory standards essential for the secure movement of potentially hazardous materials like ammunition. The form includes vital details such as the vehicle's specifications and compliance with Army regulations, which are critical to ensuring the safe and legal transportation of ammunition.

In contrast, the other forms listed serve different purposes. DA Form 3151 is used for the turn-in of property, particularly for inventory control. DA Form 581 pertains to requisitioning and issue of ammunition, documenting the request for or distribution of ammunition rather than transport qualifications. DA Form 1687, on the other hand, is an authorization signature card for individuals who can receive property, and does not relate to vehicle safety or transport specifications. Thus, DD Form 626 is the appropriate form for the requirement specified in the question.