The correct answer is: d) Monsoon.
The monsoon season is characterized by heavy rainfall and strong winds. It is caused by the difference in temperature between the land and the ocean. In the summer, the land heats up faster than the ocean, causing the air to rise. This creates an area of low pressure over the land. The air from the ocean, which is cooler and has a higher pressure, then flows in to fill the low pressure area. This flow of air is what causes the monsoon winds.
The monsoon winds bring with them a lot of moisture from the ocean. This moisture condenses into clouds and falls as rain. The monsoon season is the wettest time of year in many parts of the world, including Assam.
The other options are incorrect because they are not characterized by heavy rainfall. Summer is the hottest time of year, but it is not usually the wettest. Winter is the coldest time of year, and it is usually the driest time of year. Spring is the time of year when the weather starts to warm up, but it is not usually as wet as the monsoon season.