When the lemon is cut, it releases gaseous molecules into the surrounding air which are responsible for the lemon's smell. These molecules move slowly at room temperature and the distance that they must cover, in relevance to their size, is immense. These reasons contribute to the fact that the smell takes a while to reach the other side of the room.
For a concave mirror, the radius of curvature is twice the focal length of the mirror:

where f, for a concave mirror, is taken to be positive.
Re-arranging the formula we get:

and since the radius of curvature of the mirror in the problem is 24 cm, the focal length is
Assuming all three bulbs are connected properly with their conductivity creating a circuit, if one is unscrewed all bulbs would shut off causing zero emission of light to be displaced. Meaning answer D would be the closest to the right response.
Hope this helps! :)