In the adjacent diagram, CP represents a wavefront and AO & BP, the corresponding two rays. Find the condition on $\mathrm{\theta }$ for constructive interference at P between the ray BP and reflected ray OP 
 

• A

  $\cos \mathrm{\theta }=3\mathrm{\lambda }/2\mathrm{d}$

• B

  $\cos \mathrm{\theta }=\mathrm{\lambda }/4\mathrm{d}$

• C

  $\sec \mathrm{\theta }-\cos \mathrm{\theta }=\mathrm{\lambda }/\mathrm{d}$

• D

  $\sec \mathrm{\theta }-\cos \mathrm{\theta }=4\mathrm{\lambda }/\mathrm{d}$