The correct answer is: B. the same operation r.p.m.
The essential condition for parallel operation of two D.C. generators is that they have the same speed. This is because the output voltage of a D.C. generator is proportional to its speed. If the two generators have different speeds, their output voltages will also be different, and this can cause problems. For example, if one generator has a higher output voltage than the other, it will try to force current into the other generator. This can damage the generators and cause them to overheat.
The other options are not essential conditions for parallel operation of two D.C. generators. For example, the two generators can have different kW ratings, as long as they are both capable of supplying the required load. They can also have different drooping voltage characteristics, as long as the drooping characteristics are compatible. And they can have different percentage regulation, as long as the percentage regulation is within acceptable limits.
In conclusion, the essential condition for parallel operation of two D.C. generators is that they have the same speed.