Answer:
b. 6,561
Step-by-step explanation:
You want to know the number of 4-digit numbers that do not contain 0.
Each of the digits may have any of 9 values. The number of possibilities is the product of the number of possibilities for each of the digits:
9×9×9×9 = 6561
The number of possible 4-digit PINs is 6561 if none of the digits is 0.