The correct answer is: A. both D.C. and A.C.
A dynamometer wattmeter is a type of wattmeter that can be used to measure both direct current (DC) and alternating current (AC) power. It consists of two coils, a fixed field coil and a movable armature coil. The fixed field coil is connected to a voltage source, and the movable armature coil is connected to a current source. When current flows through the armature coil, it creates a magnetic field that interacts with the magnetic field of the fixed field coil. This interaction produces a torque that causes the armature coil to rotate. The amount of rotation is proportional to the power being measured.
Dynamometer wattmeters are accurate and reliable, and they can be used to measure a wide range of power levels. They are commonly used in industrial and commercial applications.
Option B is incorrect because dynamometer wattmeters can be used to measure AC power. Option C is incorrect because dynamometer wattmeters can be used to measure DC power. Option D is incorrect because dynamometer wattmeters are not suitable for measuring all types of power.