The correct answer is A. Low.
The starting resistance of a D.C. motor is generally low. This is because the motor is designed to start quickly and easily. The low resistance allows the current to flow freely, which helps the motor to start up quickly.
Option B is incorrect because 500 ohms is a relatively high resistance. This would make it difficult for the motor to start up quickly.
Option C is incorrect because 1000 ohms is also a relatively high resistance. This would make it even more difficult for the motor to start up quickly.
Option D is incorrect because infinitely large resistance would prevent any current from flowing through the motor. This would make it impossible for the motor to start up.
In conclusion, the starting resistance of a D.C. motor is generally low. This is because the motor is designed to start quickly and easily. The low resistance allows the current to flow freely, which helps the motor to start up quickly.