Pollen grains are stored for many years in the liquid nitrogen, having the temperature of -196 degrees of Celsius. This method of saving is called cryopreservation. The low temperature allows storing for a longer period as it reduces the growth rate of cells. The cryoprotective agents delay the aging of the plants and protect the plants from the damages due to cold.
So, the correct answer is 'Liquid nitrogen (-196o C)'.