Can you find a way pof restating this question? There is no single DIGIT that answers this question but there might be a 3 or 4 digit number

What do you mean? Do you have to enter a number to the end of, say, 42 and create a three digit number divisible by 9?

If so, note that a number is divisible by 9 if the sum of its digits is divisible by 9.

Thus, if we had 3 to 42 to get 423, the sum of the digits (4+2+3=9) is divisible by 9, therefore, so is 423 divisible by 9.