The answer is (D), the oscilloscope.

An oscilloscope is an electronic instrument, which is used for graphical representation of electrical signals that vary over time.

In an oscilloscope, the values of the electric signals are displayed on a screen with coordinates (X, Y), where typically the horizontal axis (x) represents time and the vertical axis (y) represents the voltage. The voltage measured at the vertical axis represents the signal amplitude. With the time measured in the x axis you can know the frequency of the signal based on the formula f = 1/T, where T is the signal period.