Introduction:
In a DC shunt generator, the terminal voltage is the voltage available at the output terminals of the generator. It is the voltage that is supplied to the load connected to the generator. Several factors can affect the terminal voltage of a DC shunt generator, including commutation, armature reaction, and armature ohmic losses. However, commutation is the only factor that cannot reduce the terminal voltage.

Explanation:
1. Commutation:
Commutation is the process by which the current flowing through the armature coils is switched from one coil to another as the rotor rotates. It ensures that the output voltage remains constant and the generator operates as a DC generator. However, commutation does not affect the terminal voltage directly. It is primarily concerned with maintaining the desired polarity of the generated voltage across the brushes.

2. Armature Reaction:
Armature reaction refers to the magnetic field produced by the armature current, which interacts with the main magnetic field produced by the field windings. This interaction can distort the main magnetic field and affect the terminal voltage. If the armature reaction is demagnetizing, it can reduce the terminal voltage. However, if the armature reaction is cross-magnetizing, it can increase the terminal voltage.

3. Armature Ohmic Losses:
Armature ohmic losses occur due to the resistance of the armature windings. These losses cause a voltage drop across the armature windings, which reduces the terminal voltage. The magnitude of the voltage drop depends on the current flowing through the armature and the resistance of the armature windings.

4. Summary:
- Commutation does not directly affect the terminal voltage of a DC shunt generator.
- Armature reaction can either increase or decrease the terminal voltage, depending on whether it is demagnetizing or cross-magnetizing.
- Armature ohmic losses always reduce the terminal voltage by causing a voltage drop across the armature windings.

Therefore, the correct answer is option 'A': Commutation cannot reduce the terminal voltage of a DC shunt generator.