The radius of curvature of the curved surface of a Plano-convex lens is 20 cm. If the refractive index of the material of the lens be 1.5, it will
Given,
Refractive index of the material, 
We know that the focus of a lens is given by:
where f is the focal length of the lens and R1 and R2 are the radius of the refracting surface on the side of the object and on the other side respectively.
When the object is on the curved side,
Substituting the values, we get
As the focal length is positive, the lens acts as a convex lens.
Now, if the object is at the plane side of the lens,
As the focal length is positive, the lens acts as a convex lens.
Hence, in both cases the lens is convex. The only option that captures this fact for both cases is (c). Thus, (c) is the correct option.