If the Y-axis represents acceleration, the most appropriate choice for the X-axis would be "c force" (which likely refers to "force" or "net force"). This is because, according to Newton's second law of motion (F = ma), acceleration is directly proportional to the net force acting on an object and inversely proportional to its mass.
The other options—speed, mass, and acceleration—would not logically be placed on the X-axis in this context. Speed is a different kinematic quantity, mass is not directly represented on a graph where acceleration is plotted against force without additional considerations, and putting acceleration on the X-axis would be redundant since it is represented on the Y-axis.