The correct answer is D.
The three components: Filter, Regulator and Lubricator (known also as the FRL unit) are designed to treat the air leaving the compressor before entering the system.
The filter strains the air of small particles and contaminations, the regulator supplies air at a determined, constant pressure, and the lubricator creates a mixture of air and oil mist to lubricate the system’s parts, thus improving their functioning and extending their service life.
The filter must be installed before the regulator, which, in turn, must be installed before the lubricator (so that the contaminating particles or lubricant oil particles do not interfere with the regulator’s action) . The filter is also installed before the lubricator so that the oil particles are not filtered from the air.