D: Ultraviolet Rays is the answer hope that helps
the answer your looking for is Optical instrument.
The expression for the magnitude of the electric field between two uniform conducting plates is

Here, V is potential difference between plates and d is separation between plates.
As the potential 6.00 cm from the zero volt plate (and 4.00 cm from the other) is 420 V.
Therefore,

Thus, the electric field strength between the plates is 7000 V/ m