Let the molar solubility of MY be $s_1$ and that of $NY_3$ be $s_2$. For the salt MY: $MY \rightleftharpoons M^+ + Y^-$ $K_{sp} = [M^+][Y^-] = s_1 \times s_1 = s_1^2$ $s_1 = \sqrt{K_{sp}} = \sqrt{6.2 \times 10^{-13}} = \sqrt{62 \times 10^{-14}} \approx 7.87 \times 10^{-7}\text{ M}$.

For the salt $NY_3$: $NY_3 \rightleftharpoons N^{3+} + 3Y^-$ $K_{sp} = [N^{3+}][Y^-]^3 = s_2 \times (3s_2)^3 = 27s_2^4$ $s_2 = \left(\frac{K_{sp}}{27}\right)^{1/4} = \left(\frac{6.2 \times 10^{-13}}{27}\right)^{1/4} \approx 3.89 \times 10^{-4}\text{ M}$.

Comparing the two solubilities, $s_1 < s_2$. Therefore, the molar solubility of MY in water is less than that of $NY_3$. Additionally, the addition of KY to the solutions introduces a common ion ($Y^-$), which will decrease the solubility of both salts due to the common ion effect (Le Chatelier's principle), making options B and C incorrect.