The correct answer is C) the result of the height of the gravity tank above the manifold. In a gravity feed lube oil system, the oil supply pressure to the main lube oil header is determined by the static head pressure created by the height of the gravity tank above the manifold. This static head pressure is the driving force that pushes the oil through the system, without the need for a service pump. The other options are incorrect because: A) and B) involve the service pump discharge pressure, which is not a factor in a gravity feed system. D) is incorrect because the static heads do not cancel out in a gravity feed system - the height of the tank above the manifold is the key factor.