Answer : (a) 30º
∠OCD = 90° (Tangent CD ⊥ Radius OC)
∠OCA = ∠OAC = 30° (OA = OC, radii)
∠ACD = ∠OCD + ∠OCA = 90° + 30° = 120°
∠BAC = 180° – 120° = 60° (Given ∠ACD and ∠BAC are supp.)
⇒ ∠BCD = ∠BAC = 60° (Angles in alternate segment are equal)
∠OCB = ∠OCD – ∠BCD = 90° – 60° = 30°