Solubility of group 2 hydroxides increases down the group

Steps:

• Group 2 oxides react with water to form hydroxides: MO + H₂O → M(OH)₂.
• Solubility of M(OH)₂ follows the order Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂.
• Higher solubility yields greater [OH⁻] in saturated solution.
• Greater [OH⁻] results in higher pH.

Why A is correct:

• Ba(OH)₂ has the highest solubility among group 2 hydroxides (solubility product Ksp decreases down the group), producing the most [OH⁻].

Why the others are wrong:

• B. Ca(OH)₂ is less soluble than Ba(OH)₂, yielding lower [OH⁻].
• C. Mg(OH)₂ is the least soluble, yielding the lowest [OH⁻].
• D. Sr(OH)₂ is less soluble than Ba(OH)₂, yielding lower [OH⁻].

Final answer: A