An unexpected stroke of good luck

A. Fortune

B. Windfall

C. Boon

D. Breakthrough

Answer: Option B

Solution(By Examveda Team)

One word substitute is Windfall.

Fortune: chance or luck as an arbitrary force affecting human affairs.
Windfall: a large amount of money that is won or received unexpectedly.
Boon: a thing that is helpful or beneficial.
Breakthrough: a sudden, dramatic, and important discovery or development.