In situation (a), when the driver steps on the brake, it applies an unbalanced force to the car, causing it to slow down or decelerate.
In situation (d), when an object is dropped vertically toward the ground, gravity acts as an unbalanced force on the object, causing it to accelerate downward. reprhase like an 8th grader
In situation (a), when you press the brake in a car, it makes the car slow down because the force from the brake is stronger than the force moving the car forward.
In situation (d), when you drop something, gravity pulls it down because it's a stronger force than anything pushing the object up.