The correct answer is D: If the throttle is manually moved from its 'zero' position, the resulting effect will tend to override the output of the governor, and secure the air to the control circuit. This is correct because the primary purpose of the pneumatic component shown in the illustration is to prevent the governor from affecting the engine's operation when the throttle is manually moved. By securing the air to the control circuit, the valve overrides the governor's output and allows the operator to have direct control over the engine, which is essential for safe operation. The other options are incorrect because they do not accurately describe the primary function of this pneumatic component. Option A discusses the valve's behavior in different positions, option B mentions transient signal transmission, and option C talks about segregating terminal signals, none of which are the main purpose of this valve.