Which method of storage is used for preserving pollen grains?

The correct answer is Option (3) → Liquid nitrogen at -196°C

Pollen grains stay alive (viable) for only a limited time after they are shed: some for about 30 minutes, others for a few months. To keep them for years, they are stored in liquid nitrogen at –196°C. At this very low temperature their life processes almost stop. Such stored pollen can be kept in pollen banks, like seed banks, and used in crop breeding programmes.

NCERT Extract:
Stamen, Microsporangium and Pollen Grain – “… You may have heard of storing semen/sperms of many animals including humans for artificial insemination. It is possible to store pollen grains of a large number of species for years in liquid nitrogen (–196°C). Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.”