Final answer:
The hydrostatic force in an underwater tunnel is the force exerted by the water on the tunnel walls, acting perpendicular to the surface due to hydrostatic pressure.
Step-by-step explanation:
The hydrostatic force in an underwater tunnel is the force exerted by the water on the tunnel walls, which is one of the options given in the student's question. This force is a result of the hydrostatic pressure caused by the weight of the water above the structure, and it acts perpendicular to the surfaces in contact with the water. Fluid pressure, being a scalar quantity, exerts force in a direction perpendicular to any surface it encounters, as evidenced in the behavior of a tire or a swimmer submerged in water, as well as against the walls of an underwater tunnel.