Bandwidth and frequency domain
You have to think about what sine waves would make that waveform, if combined with the right amplitude and phase.

Assuming f0 is the fundamental signal frequency of 1kHz (1ms period) if it had no harmonics it would look like this:-



Now add 3f0 (3kHz) with the same phase but 30% amplitude, and you get this:-



Notice how the humps are going up and down at 3f0. Still not there yet...

Finally, add 5f0 (5kHz) at 30% amplitude:-



Bingo! We have a match. The correct answer must be the middle graph
because it is the only one with f0, 3f0, and 5f0.  

The bandwidth is the range from lowest to highest frequency which is present in the signal, in this case 1kHz to 5kHz = 4kHz.