A, B, C and D elements form compounds AC, ${\mathrm{A}}_2\mathrm{D}$ and BD. If AC and ${\mathrm{A}}_2\mathrm{D}$ are radioactive and BD is not radioactive compound, then which among the following pair of compounds may not be radioactive: 

It means an element is common in both of them which is causing the radioactivity. In AC, and ${\mathrm{A}}_2\mathrm{D}$, element A is common so, it is confirm that element A is radioactive and element C and D can be radioactive or cannot be. It cannot be said clearly. So, the compound having element A will be radioactive.