To find the largest number formed by 1,3,9 and 5, arrange the digits in descending order.
9 > 5 > 3 > 1. So the largest number is 9531.
To find the smallest number formed by 1,3,9 and 5, arrange the digits in ascending order.
1 < 3 < 5 < 9. So the smallest number is 1359.
The difference between them is,
9531 - 1359 = 8172
Hence option 'a’ is correct.