Why do mosses need water for fertilization

Becasue they need to stay hydrated so they won't dry out or die.

Mosses are types of bryophytes, meaning they’re a part of a group of plants that NEED moist environments to reproduce. This is because their flagellated (whip like moving) sperm must swim through water to reach the egg.

Ibelieve the answer is relative humidity  because it makes more sense that way. solar flares usually occur in space it doesn't affect the earth, well it could but it wont make a significant damage.  latitude is land it doesnt do anything. prevailing winds, i dont know the correct meaning for this but i doubt that affects the land so it has to be relative humidity.