Net dipole moment $\mu$=2(bond moment)$\times \cos \large\frac{\theta}{2}$
But given $AX_2$ is a non polar molecule.Hence $\mu=0$
$0=2(0.56)D\times \cos\large\frac{\theta}{2}$
$\cos\large\frac{\theta}{2}$$=0$
$\theta=180^{\large\circ}$
Hence (a) is the correct answer.