(a) A polaroid consists of long chain molecules aligned in a particular direction. The electric vectors along the direction of the aligned molecules get absorbed. So, when an unpolarised light falls on a polaroid, it lets only those of its electric vectors that are oscillating along a direction perpendicular to its aligned molecules to pass through it. The incident light thus gets linearly polarised.
Whenever unpolarised light is incident on the boundary between two transparent media, the reflected light gets partially or completely polarised. When reflected light is perpendicular to the refracted light, the reflected light is a completely polarised light.
(b) Let the angle between the pass axis of A and C = θ
Intensity of light passing through A =I0/2
The third polaroid is placed at θ = 45°.