The equivalent weight of a metal in different compounds are $18.6$ ad 28. Atomic mass of the metal would be

Equivalent weight of a metal $=\frac{atomicweight}{n -factor/valency}$

If we take the ratio of the 2 equivalent weights,

So, we can see that the ratio of valencies of the metal in these compounds is $2:3$. Now, from above formula atomic weight $=$ Equivalent weight $x$ valency

Hence, option (D) is correct.