Any source of light, opaque object and screen are necessary to form a shadow.

● Option (a) : the sun is not necessary to form a shadow because we can form shadow from some other sources of light also like torch etc. Hence the answer is option (a).

● Option (b) : screen is necessary to take the image.

● Option (c) : any source of light is necessary to form the shadow like torch, bulb, streetlight etc.

● Option (d) : the opaque object is necessary to block the light.