$A$ beam with wavelength $\lambda$ falls on a stack of partially reflecting planes with separation $d$. The angle $\theta$ that the beam should make with the planes so that the beams reflected from successive planes may interfere constructively is (where $n = 1, 2, \dots$)

• A
  $\sin^{-1}\left(\frac{n\lambda}{d}\right)$
• B
  $\tan^{-1}\left(\frac{n\lambda}{d}\right)$
• C
  $\sin^{-1}\left(\frac{n\lambda}{2d}\right)$
• D
  $\cos^{-1}\left(\frac{n\lambda}{2d}\right)$