A) All the following are correct.
- To allow for calibration error in the transmitter frequency display.
- So that modulation sidebands do not extend beyond the band edge
- To allow for transmitter frequency drift
Note from NJ2X: You definitely do not want to transmit outside the amateur radio bands since it is illegal. However, this is exactly what will happen if you are transmitting on a frequency that is at the very edge of one of the amateur radio bands. That is because all modes of communication use a certain amount of bandwidth so your radio is transmitting over a frequency range and not just the frequency you are tuned to.
For example, when transmitting on upper side band on 28.500Mhz the frequency range would extend up to 28.500Mhz + 3Khz. Technician privileges end at 28.500Mhz so any voice transmission above this frequency would be illegal if made by a technician license holder.