Armature reaction of an unsaturated D.C. machine is

The correct answer is A. Cross-magnetizing.

Armature reaction is the effect of the armature current on the magnetic field of a DC machine. The armature current creates a magnetic field that opposes the main field of the machine. This is called cross-magnetizing.

The armature reaction can cause a number of problems, including:

• A decrease in the output voltage of the machine.
• A decrease in the speed of the machine.
• A decrease in the efficiency of the machine.

The armature reaction can be minimized by using a compensating winding. A compensating winding is a winding that is placed on the stator of the machine and is designed to cancel out the effect of the armature current.

Here is a brief explanation of each option:

• Option A: Cross-magnetizing. This is the correct answer. The armature current creates a magnetic field that opposes the main field of the machine. This is called cross-magnetizing.
• Option B: Demagnetizing. This is not the correct answer. The armature current does not demagnetize the machine.
• Option C: Magnetizing. This is not the correct answer. The armature current does not magnetize the machine.
• Option D: None of above. This is not the correct answer. The armature reaction is a real phenomenon that can have a significant impact on the performance of a DC machine.