Answer:
d
Explanation:
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When an electron stops, it emits a photon with energy equal to the kinetic energy lost by the electron:

The energy of the photon is

where

is the Planck constant and f is the frequency. Therefore, the maximum frequency of the emitted photon occurs when the loss of kinetic energy is maximum.
The maximum loss of kinetic energy of the electron occurs when the electron stops completely, so it loses all its energy:

Keeping in mind that

, we have

And so, this corresponds to the energy of the emitted photon, E. Therefore, we can find the maximum frequency of the emitted photon:
Given there is a standing wave a 200 and 250Hz, and there are none between 250 and 300Hz, we know that the difference in frequency between each harmonic is 50Hz.
Therefore, the fundamental frequency (the lowest that can be achieved), is 50Hz, as it is 0+the difference in frequency between each harmonic.
The third harmonic is 3x50 = 150Hz, as the 1st harmonic is the fundamental frequency, the 2nd is the fundamental frequency plus the difference between each harmonic and so on.