**Answer:** Option **
A **

**Explanation:**

OA = OB = OC = OD = r units (radii)

AB = a metre and CD = b metre

∠AOB = 60° and ∠COD = 90°

In ΔCOD, By pythagoras theorem

CD

^{2} = OC

^{2} + OD

^{2}
b

^{2} = r

^{2} + r

^{2} = 2r

^{2} ...(i)

In ΔAOB,

OA = OB

∴ ∠ABO = ∠OAB

∠AOB + ∠ABO + ∠OAB = 180°

60° + ∠OAB + ∠OAB = 180°

2∠OAB = 180° – 60° = 120°

∠OAB = 60° = ∠ABO

∴ ΔAOB is an equilateral triangle.

OA = OB = AB ⇒ a = r

From equation (i),