Hydraulic brakes are based on

Explanation:
Hydraulic brakes work on the principle of Pascal’s law.
 According to this law, whenever pressure is applied on fluid it travels uniformly in all the directions. Therefore when we apply force on a small piston, the pressure gets created which is transmitted through the fluid to a larger piston.
As a result of this larger force, uniform braking is applied on all four wheels. As braking force is generated due to hydraulic pressure, they are known as hydraulic brakes.
Liquids are used instead of gas as liquids are incompressible.

Final answer:
Hence, the correct option is A, i.e. Pascal’s law.