The correct answer is option 4 - upward force.

Buoyancy refers to the upward force exerted by a fluid (such as water or air) on an object immersed in it. This force is directed opposite to the gravitational force pulling the object downward.

Option 1 - upward pressure: Upward pressure alone cannot explain buoyancy as pressure is a scalar quantity and does not have a specific direction.

Option 2 - downward pressure: Buoyancy works in the opposite direction of pressure, so it cannot be described as a downward pressure.

Option 3 - downward force: This is incorrect as buoyancy is an upward force, not downward.

Option 4 - upward force: This is the correct option. Buoyancy acts as an upward force that opposes gravity and enables objects to float or become less submerged in a fluid.