Final answer:
The constant of proportionality 'k' in this scenario is velocity.
Step-by-step explanation:
The constant of proportionality k in this scenario is velocity. This is because velocity is the rate at which an object moves in a particular direction. In the given situation, the distance covered by the car is directly proportional to the time it moves, meaning that if the time doubles, the distance also doubles. Therefore, the constant of proportionality in this case is velocity.