Solution:

The correct answer is Option D: All of the above.

Explanation:
Let's go through each option to understand how the power brake can be exerted:

A. Electrical energy:
Some modern vehicles use electrically assisted braking systems, where electrical energy is used to enhance braking force. This can be achieved through technologies like regenerative braking or electrically operated brake boosters.

B. Engine vacuum:
Many vehicles utilize engine vacuum to assist in applying the brakes. This is commonly known as a vacuum booster or vacuum-assisted brake system. The engine's vacuum creates a pressure differential, which helps in applying the brakes with less pedal effort.

C. Air pressure:
Air pressure is commonly used in large commercial vehicles, such as trucks and buses, for their braking systems. These vehicles use an air brake system, where compressed air is used to exert force on the brake mechanisms.

Therefore, as all of the options mentioned in the question (electrical energy, engine vacuum, and air pressure) can be utilized to exert power for the brake system, the correct answer is Option D: All of the above.