JEE Main Chemistry question

The concentration of hydroxyl ion in a solution left after mixing 100 mL of 0.1 M MgCl₂ and 100 mL of 0.2 M NaOH [Ksp of Mg(OH)₂=1.2×10⁻¹¹] is:

• (A)2.8×10⁻³
• (B)2.8×10⁻²
• (C)2.8×10⁻⁴Correct
• (D)2.8×10⁻⁵