For this problem you’ll need to use hypothetical numbers to test the statements. D(Dn) = n does not always hold true because of leading zeros. D(D30) becomes 3. III does not always hold true either because D(1+12) = 31, which is not equal to 1+D12, which is 22. Only II holds true always because D(10*n) will always have one less digit than 10 * n because the leading zero will be lost. For instance, D(10*3) becomes 3, will is less than 10*3, or 30. C is therefore the correct answer choice.