The correct answer is: a) June to September.
The southwest monsoon winds are a seasonal wind that blows from the southwest to the northeast across the Indian subcontinent. They bring significant rainfall to Assam from June to September. The monsoon winds are caused by the difference in temperature between the land and the ocean. During the summer, the land heats up faster than the ocean, creating an area of low pressure over the land. This low pressure draws in the moist southwesterly winds from the ocean. The monsoon winds bring heavy rains to Assam, which is essential for the state’s agriculture.
The other options are incorrect because they do not correspond to the time period when the southwest monsoon winds bring significant rainfall to Assam. Option b) is incorrect because the southwest monsoon winds do not bring significant rainfall to Assam from March to May. Option c) is incorrect because the southwest monsoon winds do not bring significant rainfall to Assam from December to February. Option d) is incorrect because the southwest monsoon winds bring significant rainfall to Assam from June to September.