Which of the following is a common fluid used in hydraulic systems?

Mineral oil is a common fluid used in hydraulic systems primarily due to its excellent lubricating properties and its ability to reduce friction between moving components. It has a high viscosity index, which means it maintains its viscosity over a wide range of temperatures, ensuring consistent performance in various environmental conditions. Additionally, mineral oil has good thermal stability, which helps to prevent the breakdown of the fluid under high temperatures and stress, and it also offers effective corrosion resistance for hydraulic components.

While air is used in pneumatic systems and water can be used in some specialized hydraulic applications, mineral oil is preferred in most hydraulic systems because it provides a balance of performance, cost, and availability. Alcohol, on the other hand, is less common for hydraulic applications due to issues with lubrication and performance at varying temperatures. Therefore, mineral oil is the standard choice for hydraulic fluid in a wide array of systems, making it the best answer for this question.