Property of all alkaline earth metals that increase with their atomic number is:
Solubility of their carbonates
Thermal stability of their sulphates
Ionisation energy
Electronegativity
(A), (C) and (D) decrease down the group but (B) increases down the group with increasing metallic character. On moving down the group, the electropositive character of alkaline earth metals increases. Due to this, their thermal stability also increases.
Therefore, option(2) is correct.