(c) mg cosθ
Explanation:
See the figure on the right. Weight of the block is mg downwards. We resolve it in two components-along the plane and perpendicular to the plane. Former component does not exert force on the plane while the later component mg.cosθ is the force which the block exerts on the plane. If we take it as action then equal and opposite force mg.cosθ is exerted by the plane on the block as reaction satisfying the "Newton's Third law of Motion".