[" A lens having focal length "f" and aperture of "],[" diameter "d" forms an image of intensity "I],[" Aperture of diameter "(d)/(2)" in central region of "],[" lens is covered by a black paper.Focal length "],[" of lens and intensity of image now will be "],[" respectively "],[[" (a) "f" and "(1)/(4)," (b) "(3f)/(4)" and "(I)/(2)],[" (c) "f" and "(3I)/(4)," (d) "(f)/(2)" and "(I)/(2)]]