How does sampling a frequency result in a duplicated spectrum? This is probably a simple question, but I have a hard time understanding how c. converts to d. in the following picture. 

Further reading as an answer is acceptable. After all, I am only asking this because the PDF I’m reading doesn't really explain why c. results in d. The more I look at it, the more I am confused.
if you consider the spectrum of analog signal as the figure in your picture.Sampling in time domain is same as multiplying a pulse train to  the analog signal.This means that the frequency response of pulse train is convoluted in frequency domain. The frequency response of pulse train is again a pulse train,hence the spectrum gets duplicated in frequency domain due to its convolution with the pulse train.