The correct answer is A) spring force. Differential needle valves used in fuel injectors are directly closed by spring force. The spring provides the necessary force to keep the needle valve closed when the fuel pressure is low or not present. When the fuel pressure increases, it overcomes the spring force, opening the valve and allowing fuel to be injected into the engine cylinder. The other options are incorrect because: B) firing pressure would not directly close the needle valve, C) cam action is used to open and close the valve, not directly close it, and D) fuel oil pressure alone does not directly close the valve - the spring force is the primary closing mechanism.