(C) The brightness of the image will reduce.
When the upper half is covered, fewer light rays pass through the lens, so the image is complete but less bright (dimmer).
A lens forms an image using all light rays passing through it. Covering half the lens reduces the number of rays, but the remaining half still refracts light to form a complete image of the whole object — just with reduced intensity/brightness. Size is unaffected, and both halves of the image are still formed. This is a standard NCERT concept (Q9 in Exercises).