Final answer:
The closest synonym for 'windfall' is 'bonanza', which also means a sudden, unexpected profit or gain.
Step-by-step explanation:
The nearest synonym for the word 'windfall' is b) Bonanza. A windfall refers to a piece of unexpected good fortune, typically an amount of money gained unexpectedly or a sudden profit. Therefore, among the options provided, 'misfortune', 'catastrophe', and 'calamity' are not correct as they imply negative events or losses. The word bonanza aligns with 'windfall', as it also signifies a large amount of something desirable, usually referring to a sudden profit or gain.