I have four guesses so far:


  none, "on", "for", and "by".



Question: How replicas are formed in Frequency domain when a signal is sampled in Time Domain?
Answer: Sampling in one domain implies periodicity in the other. For example the Discrete Fourier Series (which the FFT is a special case off), requires both time and frequency domain signals to be discrete and periodic. 

This really isn't a topic that can be exhaustively discussed on board like this. I would recommend spending some quality time with good text book such as this http://www.amazon.com/Digital-Signal-Processing-Alan-Oppenheim/dp/0132146355 or http://www.dspguide.com/