The correct answer is C) The viscosity of the fluid is relatively unimportant in the pump operation. Spur gear pumps are designed to handle a wide range of fluid viscosities, from light oils to heavy lubricants. The operation of a spur gear pump is primarily dependent on the displacement of the fluid between the meshing gears, rather than the viscosity of the fluid. As long as the fluid can flow into the pump and through the gears, the pump will operate effectively regardless of the fluid's viscosity. The other answer choices are incorrect because spur gear pumps are not limited to only high viscosity fluids (A) or light oils (B), and they can handle fluids more viscous than lubricating oil (D).