Choose the number-pair that is different from the other three.

Logic: Multiply the first of each pair by 2 and then add 4 to get the second number.

First option: 0 : 6 $$\rightarrow$$ (0 $$\times$$ 2) + 4 = 0 + 4 = 4 $$\neq$$ 6​

Second option: 5 : 14 $$\rightarrow$$ (5 $$\times$$​ 2) + 4 = 10 + 4 = 14​

Third option: 12 : 28 $$\rightarrow$$ (12 $$\times$$​ 2) + 4 = 24 + 4 = 28​

Fourth option: 21 : 46 $$\rightarrow$$ (21 $$\times$$​ 2) + 4 = 42 + 4 = 46​

So, only the first option does not follow the same pattern as the others.

Thus, the correct answer is (a).