A contact force is a force that acts on an object through direct physical contact. In the options you've provided:
A. A person standing perfectly still on the ground - This represents a contact force because the person is exerting a normal force on the ground, and gravitational force acts on them through contact with the ground.
B. Rain falling from the sky - This is not a contact force as it involves the force of gravity acting at a distance.
C. A magnet pulling a screw toward it - This is also not a contact force; instead, it involves a magnetic force, which acts at a distance.
D. A skydiver in freefall - This scenario does not involve contact forces, as the skydiver is falling under the influence of gravity.
Therefore, the correct answer is A. a person standing perfectly still on the ground.