An unexpected receipt of money is called a windfall.
Option D
Step-by-step explanation:
An unexpected gain in income can be termed as a windfall gain. The source such quantum payment might be due to several reasons like winning a lottery, unanticipated property inheritance, bonus payments etc. Probability of such payments tends to be lesser as, people normally do not gain from such sources until luck runs in their favor or they work day in and day out without expecting financial benefits in return or trying out their luck by enrolling in lottery systems.