An alkaline earth metal ‘$\mathrm{M}$’ readily forms water-soluble sulphate and water-insoluble hydroxide. Its oxide $\mathrm{MO}$ is very stable to heat and does not have a rock-salt structure. $\mathrm{M}$ is

The correct option is B

$\mathrm{Be}$

Explanation for correct option:

(B) $\mathbf{Be}$

1. As $\mathrm{Be}$ is smaller is size so it forms water-soluble sulphate and water insoluble hydroxides.
2. Oxides of $\mathrm{Be}$ are very stable to heat due to increase in degree of polymerisation.
3. $\mathrm{Be}$ do not have rock salt structure because it form sodium beryllate on dissolving in sodium hydroxide.

Explanation for incorrect option:

(A) $\mathbf{Ca}$

1. Sulphates of calcium are insoluble in water due to high hydration enthalpy.
2. Hydroxides of calcium are also slightly soluble in water.
3. Hence $\mathrm{Ca}$ is not metal $\mathrm{M}$ .

(C) $\mathbf{Mg}$

1. $\mathrm{Mg}$ does not react with sodium hydroxide to form salt.
2. Hence $\mathrm{Mg}$ is not metal $\mathrm{M}$ .

(D) $\mathbf{Sr}$

1. As $\mathrm{Sr}$ is larger in size so it forms water insoluble sulphates and slightly soluble hydroxides.
2. Hence $\mathrm{Sr}$ is not metal $\mathrm{M}$ .

Hence option B is correct, $\mathrm{Be}$ is metal $\mathrm{M}$.