The most powerful electromagnet in a normal DC machine operating at full load conditions is the field winding (option A). Let's understand why this is the correct answer by examining the working principle and components of a DC machine.

Working Principle of a DC Machine:
A DC machine works on the principle of electromagnetic induction. It converts mechanical energy into electrical energy (generator mode) or vice versa (motor mode) using the interaction between the magnetic field and the current-carrying conductors.

Components of a DC Machine:
1. Field Winding: It is a coil of wire wound around the pole pieces of the machine. The field winding produces the magnetic field necessary for the operation of the machine. It can be either a shunt, series, or compound winding, depending on the machine's design.

2. Armature Winding: It is a set of conductors that are mounted on the rotor or the armature of the machine. When the armature winding carries current, it interacts with the magnetic field produced by the field winding, resulting in the generation of a mechanical force (motor mode) or an induced voltage (generator mode).

3. Interpole Winding: It is an additional winding placed between the main poles of the machine. The interpole winding helps in reducing the armature reaction and prevents commutation problems in the machine. It produces a magnetic field that counteracts the armature field distortion.

4. Compensating Winding: It is another additional winding used to reduce the cross-magnetizing effect in the machine. The compensating winding produces a magnetic field that is opposite to the armature field, compensating for its effect on the main field.

Explanation of the Answer:
In a normal DC machine operating at full load conditions, the most powerful electromagnet is the field winding. This is because the field winding is responsible for producing the main magnetic field that interacts with the armature winding. The strength of the magnetic field determines the machine's overall performance and power output.

The armature winding, interpole winding, and compensating winding do play important roles in the operation of the machine, but they do not directly influence the strength of the magnetic field. The armature winding carries the load current, inducing the mechanical force or the induced voltage. The interpole winding and compensating winding help in reducing the armature reaction and improving the commutation process.

However, it is the field winding that determines the strength of the magnetic field and, consequently, the power of the electromagnet. By controlling the current flowing through the field winding, the operator can adjust the magnetic field strength and, thus, control the machine's performance and power output.

In conclusion, the field winding is the most powerful electromagnet in a normal DC machine operating at full load conditions because it determines the strength of the magnetic field, which is essential for the machine's operation and power output.