Correct option is (d) 6-14 micron
The explanation is: Ragweed pollen (19 micron in diameter) and polystyrene latex particles (6–14 micron in diameter) seem to meet these requirements. Of the two, polystyrene latex is preferred for calibration purposes (Thom, 1972). The particles when used seldom plug the orifice. These can be conveniently obtained in the range of 5 million particles per cubic mm.
