What is the acceleration of a car that maintains a constant velocity of 100 km/h for 10 s?

'Acceleration' is the rate at which 'velocity' changes. When you say "constant velocity", the rate at which velocity is changing is zero, so that's just another way of saying "zero acceleration".

If we know that acceleration is actually the change of velocity over time, we can come to a conclosion:

A constant velocity is a velocity that doesnt change. It is constantly the same, meaning no changes in velocity. In other words, the change in velocity would be 0.

In other words, the acceleration iz zero since the velocity doesnt change.