The correct answer is: A. Increase of short-circuit current of the system
A constant voltage transmission system is a type of power transmission system in which the voltage at the receiving end is kept constant, regardless of the load. This is achieved by using a voltage regulator, which automatically adjusts the output voltage of the generator to compensate for changes in load.
One of the demerits of a constant voltage transmission system is that it can lead to an increase in the short-circuit current of the system. This is because, when the load on the system increases, the voltage regulator will increase the output voltage of the generator. This will cause the current in the system to increase, and the short-circuit current will also increase.
The other options are not demerits of a constant voltage transmission system. Option B is a benefit of a constant voltage transmission system, as it means that the voltage at all loads at the line terminals is steady. Option C is also a benefit, as it means that the line can be protected with higher terminal reactants. Option D is also a benefit, as it means that the power factor at times of moderate and heavy loads is improved.
In conclusion, the demerit of a constant voltage transmission system is that it can lead to an increase in the short-circuit current of the system.