The correct answer is D: The viscosity sensor is fitted with a constant speed fixed capacity pump and a capillary tube which produces a pressure drop at constant flow which is proportional to the oil viscosity. This is correct because the viscosity of a fluid is inversely proportional to the flow rate through a capillary tube at a constant pressure drop. Therefore, a constant speed fixed capacity pump and a capillary tube will produce a pressure drop that is proportional to the oil viscosity, as the flow rate will decrease as the viscosity increases. The other options are incorrect because they do not accurately describe the relationship between the viscosity sensor components and the resulting pressure drop. Options A and B describe a variable capacity pump, which would not produce a constant flow. Option C describes an inverse relationship between pressure drop and viscosity, which is the opposite of the correct relationship.