ILLUMINATE PROHIBITED LAMP(S)

Illuminate Prohibited Lamps(s)

In this case, the single highlighted cell draws your attention to the Prohibited Lamp Cell at C5-R3. This cell is not yet illuminated, and is not permitted to contain a lamp. It must be illuminated by a lamp in some other cell, and the only cell capable of doing this is the highlighted cell. Consequently, it must be filled with a lamp.