It is given in the question that, ABCD is a quadrilateral where AO and BO are the bisectors of ∠ A and ∠ B

We know that, sum of all angles of a quadrilateral is equal to 360^o

∴∠ A + ∠ B + ∠ C + ∠ D = 360^o

∠ A + ∠ B + 70^o + 30^o = 360^o

∠ A + ∠ B = 360^o – 100^o

∠ A + ∠ B = 260^o

1/2 (∠A + ∠B) = 1/2 × 260°

1/2 (∠A + ∠B = 130°

Now, in triangle AOB

1/2 (∠A + ∠B) + ∠AOB = 180°

130^o + ∠ AOB = 180^o

∠ AOB = 180^o – 130^o

∠ AOB = 50^o

Hence, option (B) is correct