A car traveling on a level road initially has 440 kJ of mechanical energy. After the brakes are applied for a few seconds, the car has only 110 kJ of mechanical energy. What best accounts for the missing mechanical energy? In other words, where did that missing energy go?

The missing mechanical energy becomes thermal energy in form of heat transferred to the medium (air and ground)

Explanation:

When the car applies the brakes, the friction between wheels and highway generates heat. This heat is transmitted to the air and to the ground.

Then, part of the mechanical energy of the car becomes thermal energy in form of heat.

The car, at the beginning has a energy of 440kJ, after the brakes are applied, the car has 110 kJ of energy. This means that necessarily the car loos 330 kJ of energy that became heat to the medium (air and ground)