Answer: The first law
Step-by-step explanation:
The first law states "an object will not change its motion unless a force acts on it." In this case the surfboard on top of the van was at rest and once the van started moving the acceleration of the surfboard wasn't equal to the acceleration of the van. So there was no force to keep it at the speed of the van if it wasn't strapped correctly. I hope this answers your question I did try my best.