Explanation:

Water vapour capacity increases with temperature increase. At low temperatures, such as in Tundra regions, the air will hold very less moisture as it becomes more compact. So, (d) is wrong.
Coastal regions generally have moderate temperatures and the air is already has a lot of moisture. So, (a) can’t be the answer. The case of option (b) is also similar due to high rainfall and not so high temperatures.
In, hot sub-tropical deserts, you will notice that evaporation is very fast, as the air can contain a lot of water vapour due to the aridity and high temperature. So, (a) is the answer.