A positive integer is subtracted from its square. The units digit of the result could be which of the following?

a. 3

b. 4

c. 5

d. 6

e. 7

Correct answer is D.

Look at the unit digit of a number, and ignore the other digits. We will denote other digits with dots ( ...). Let's say we have a number ... 3.

... 3^2 - 3 = ... 9 - 3 = ... 6

Check for numbers which unit digit is 1,2,4,5,6,7,8, or 9. They don't satisfy any of the answer options.