The correct option is B August to September
From June to July, the increase in rainfall is from 8 mm to 10 mm. The increase in rainfall is 2 mm.
From August to September, the increase in rainfall is from 9 mm to 31 mm. The increase in rainfall is 22 mm.
From September to November, the increase in rainfall is from 31 mm to 50 mm. The increase in rainfall is 19 mm.
The greatest increase in rainfall is observed from August to September.