(D)      Let $I_0$ be intensity of unpolarised light incident on first polaroid.

           $I_1=$ Intensity of light transmitted from $1^{st}$ polaroid $=\frac{I_0}{2}$

           $\theta$ be the angle between $1^{st}$ and $2^{nd}$ polaroid

           $\varphi$ be the angle between $2^{nd}$ and $3^{rd}$ polaroid

           $\theta +\varphi ={90}^o$ (as $1^{st}$ and $3^{rd}$polaroid are crossed)

           $\varphi =90°-\theta$

           $I_2=$ Intensity from second polaroid

           $I_2=I_1{\cos }^2\theta =\frac{I_0}{2}{\cos }^2\theta$

           $I_3=$ Intensity from $3^{rd}$ polaroid

           $I_3=I_2{\cos }^2\varphi =I_1{\cos }^2\theta {\cos }^2\varphi$

           $I_3=\frac{I_0}{2}{\cos }^2\theta {\cos }^2\varphi$

           $\varphi =90-\theta$

          $I_3=\frac{I_0}{2}{\left[\frac{2\sin \theta \cos \theta }{2}\right]}^2$

          $I_3=\frac{I_0}{8}{\sin }^{2}2\theta$

          $I_3$ will be maximum when sin $2\theta =1$

          $2\theta =90$

(1)

$I_1=\frac{I_0}{2} \text{and} I_2=I_1{\cos }^2\theta$

$=\frac{I_0}{2}{\cos }^{2}45°I_2=\frac{I_0}{4}$

(1)

By Brewster's law

At complete reflection refracted ray and reflected ray are perpendicular.