(i) 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.
Alternatively,
Whenever an unpolarised light is incident on a transparent surface, the reflected light gets partially or completely polarised/ the reflected light gets completely polarised when the reflected and refracted light are perpendicular to each other.
(ii) Let θ be the angle between the pass-axis of A and C.
Intensity of light passing through A = I0/2
Intensity of light passing through C = (I0/2) cos2θ