A screen is placed at a distance 40 cm away from an illuminated object. A converging lens is place between the source and the screen and it is attempted to form the image of the source on the screen. If no position could be found, the focal length of the lens

The distance of the object from lens is 40cm. For no position of lens D< 4f, i.e. 40<4f that means, f>10cm. Therefore, the focal length should be greater than 10cm

Hence, option B is correct.