The correct answer is D) Acts as an accumulator. The air receiver in a compressed air system functions as an accumulator, storing compressed air and helping to maintain a steady air pressure in the system. The air receiver acts as a buffer, smoothing out fluctuations in air pressure and providing a reserve of compressed air when demand increases. This helps ensure a consistent supply of air to the various pneumatic systems and components that rely on the compressed air. The other answer choices are incorrect because: A) The air purification happens upstream of the air receiver, B) The air receiver does not directly condense moisture, and C) The air receiver provides overpressure protection, but this is not its primary function.