Explanation:

- Objects submerged in a fluid experience buoyancy.

- Buoyancy is the net upward force exerted by the fluid.

- It results from pressure differences at various depths.

- Option 1: A downward force - Not applicable since buoyancy acts upwards.

- Option 2: A downward pressure - Misleading, as buoyancy deals with upward force, not pressure direction.

- Option 3: An upward force - Correct. Buoyancy acts upward, opposing gravity.

- Option 4: An upward pressure - Inaccurate, since buoyancy specifically refers to net force.