The correct answer is A) increases. With an increase in temperature, the volume of hydraulic fluid increases due to thermal expansion. This is a well-established principle in fluid dynamics and is covered in the US Coast Guard Captain's License Examinations. As the temperature rises, the molecules in the hydraulic fluid gain more kinetic energy, causing them to move further apart and occupy a larger volume. The other options are incorrect because: B) the volume would expand, not contract, with an increase in temperature; C) the volume would increase regardless of pressure changes; and D) the volume would not remain the same, but rather increase with higher temperatures.