The correct answer is: A. Runs as a generator.
When one DC motor drives another DC motor, the second DC motor will run as a generator. This is because the first DC motor will be acting as a power source, and the second DC motor will be acting as a load. The current that flows through the first DC motor will create a magnetic field, and this magnetic field will induce an emf in the second DC motor. The emf will cause a current to flow through the second DC motor, and this current will create its own magnetic field. The two magnetic fields will interact with each other, and this will cause the second DC motor to rotate.
Option B is incorrect because the second DC motor will not run as a generator if it is not excited. The second DC motor needs to be excited in order for it to generate an emf.
Option C is incorrect because the second DC motor will not run as a motor and come to stop after sometime. The second DC motor will continue to run as a generator as long as the first DC motor is providing power.
Option D is incorrect because the second DC motor will run as a generator.