Correct answer is (c) A straight line perpendicular to the plane of the plate rotates such that it remains perpendicular to the tangent to the deformed surface

To explain I would say: In the SDT, we relax the normality assumption of CPT, i.e., transverse normal may rotate without remaining perpendicular to the mid-plane. The Classical Plate Theory is based on the assumption that a straight line perpendicular to the plane of the plate is (1) inextensible, (2) remains straight, and (3) rotates such that it remains perpendicular to the tangent to the deformed surface.