Explanation:
Bragg's law is given by :
[tex]2d\ sin\theta=n\lambda[/tex]
Where
d is the lattice spacing
n is the order
[tex]\lambda[/tex] is the wavelength of x- ray
[tex]\theta[/tex] is the diffraction angle.
Diffraction angle is defined as the angle between incident beam and the reflecting planes of atom. Hence, the correct option is (b).