(a) In unpolarised light, light waves vibrate in more than one plane; whereas in linearly polarised light, vibrations of light waves occur in a single plane.
(b) A polaroid consists of long-chain molecules aligned in a particular direction. The electric vectors (associated with the propagating light wave) along the direction of the aligned molecules get absorbed. Thus, if an unpolarised light wave is incident on such a polaroid, then the light wave will get linearly polarised with the electric vector oscillating along a direction perpendicular to the aligned molecule.
(c) Unpolarized light scattering from air molecules shakes their electrons perpendicular to the direction of the original ray. The scattered light therefore has a polarisation perpendicular to the original direction and none parallel to the original direction.