(d) 35°
Since PA is a tangent, ∠OAP = 90°. ∠AOP = 180° − 125° = 55° (angles on a straight line). In △OAP: ∠APO = 180° − 90° − 55° = 35°.
Key steps: (1) Radius OA ⊥ tangent PA (Theorem 10.1), so ∠OAP = 90°. (2) ∠AOP + ∠POB = 180° (linear pair), giving ∠AOP = 55°. (3) Angle sum in △OAP gives ∠APO = 35°. Examiners expect you to state the tangent-radius perpendicularity and use the linear pair/angle sum property clearly.