Final answer:
The outward force on an air tank is balanced by an equal inward force, as per Newton's third law of motion, to keep the tank stationary and ensure a net force of zero.
Step-by-step explanation:
The outward force on one end of an air tank is due to the pressure of the fluid inside it, exerted perpendicular to the tank's inner surfaces. If the tank is not accelerating, it follows from Newton's third law of motion that this outward force must be balanced. Therefore, the correct answer to the question, "How is this force balanced?" is b) The tank experiences an equal inward force. This inward force could be provided by the structural strength of the tank itself or by external forces acting on it, ensuring that the resultant or net force on the tank is zero, in accordance with Newton's first law of motion - an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.