The "dark current" of photoreceptors refers to the exit of k+ out of the cell when light is not hitting the cell. the exit of na+ and ca2+ out of the cell when light is not hitting the cell. the entry of k+ into the cell when light is not hitting the cell. the entry of na+ and ca2+ into the cell when light is not hitting the cell.

The correct answer is the entry of Na+ and Ca2+ into the cell when light is not hitting the cell.  
In the dark, the receptor is in a depolarized state which depends on the presence of ion channels in the outer segment membrane that permit Na+ and Ca2+ ions to flow into the photoreceptor cell (depolarizing it to about −40 mV). As a result, the degree of inside negativity is reduced. High levels of cGMP in the outer segment keep the channels open.