Half of a water tank's capacity is filled with water. The tank is kept on a truck that moves at a constant speed. When the brake was suddenly applied, the water in the tank would:

Inertia is the property of an object that opposes any event that attempts to move or change the velocity of the object. Then, an object at rest would prefer to remain at rest, while an object in motion would prefer to continue moving. The greater the mass of the object, the greater the tendency to resist events.

Since the truck is moving forward at a constant speed, the tank and the water are also moving at a constant speed.

When a sudden brake is applied, the forward motion of the water in the tank is opposed.

Because of the inertia of water, it would tend to continue moving forward. As a result, when braking suddenly, it moves forward.