The pH of a 0.25 M solution of a weak monobasic base is 8.76 at 25 °C. Calculate Kb for the base.

A) 1.2 × 10⁻¹²
B) 2.5 × 10⁻¹⁰
C) 4.8 × 10⁻⁹
D) 7.1 × 10⁻⁸