A pollen _ grain germinates and develops into :

Explanation

When a pollen grain germinates, it develops into a micro gametophyte, which carries the male gametes (sperm cells) for fertilization. Therefore, the correct option is 'Micro gametophyte' as it directly relates to the development of pollen grains. The other options, Prothallus, Saprophyte, and Mega gametophyte, are incorrect because they do not represent the specific structure that pollen grains develop into during plant reproduction.