The correct answer is: A. their rising voltage characteristics
Flat and over compounded generators have rising voltage characteristics, which means that their output voltage increases as the load increases. This can lead to unstable parallel operation because the generators will try to take over from each other, causing the output voltage to fluctuate wildly.
Unequal number of turns in their series field windings, unequal speed regulation of their primemovers, and unequal series field resistances can all contribute to unstable parallel operation, but they are not the main factors.
Unequal number of turns in their series field windings can cause the generators to have different output voltages, which can lead to instability. Unequal speed regulation of their primemovers can cause the generators to have different speeds, which can also lead to instability. Unequal series field resistances can cause the generators to have different current outputs, which can also lead to instability.
However, the main factor that leads to unstable parallel operation of flat and over compounded generators is their rising voltage characteristics.