a) If the first digit of 3 digit number is 0 then the number becomes 2 digit number so the first digit is not 0
So, the smallest 3 digit number is 505.
b) The smallest 4 digit number is 2588
c) If the first digit of 4 digit number is 0 then the number becomes 3 digit number so the first digit is not 0
So, the smallest 4 digit number is 7079