Replace * by the smallest number, so that the number 54*61 is divisible by 9.

Here, Sum of the known digits of  54*61 = 5 + 4 +6+1 = 16

Now, the number next to 16, which is divisible by 9 is 18.

Thus, if we replace the * by 2, the sum of digits will become 18 and the number becomes divisible by 9.

Hence, * is replaced by 2.