The magnitude of the magnetic field (B) due to loop ABCD at the origin (O) is

Question

( A ) $\frac{\mu_o I}{4 \pi} \begin{bmatrix} \frac{b-a}{ab} \end{bmatrix}$
( B ) zero
( C ) $\frac{\mu_o(b-a)}{24ab}$
( D ) $\frac{\mu_o I}{4 \pi} [ 2 (b-a) + \frac{\pi}{3} (a+b) ]$