The correct answer is option 'B' - two.

When balancing a single disturbing mass, it is necessary to introduce balance masses in order to counteract the unbalanced forces caused by the disturbing mass. These balance masses are typically introduced in a plane parallel to the plane of rotation of the disturbing mass.

Here's a detailed explanation of why two balance masses are required:

1. Understanding unbalanced forces:
- A disturbing mass creates an unbalanced force when it rotates.
- This unbalanced force causes vibrations and potentially affects the stability and performance of the system.
- The goal of balancing is to counteract this unbalanced force.

2. Balancing principles:
- Balancing is achieved by introducing balance masses that generate equal and opposite forces to counteract the unbalanced force.
- These balance masses are typically positioned at a specific distance from the axis of rotation to create the required force.

3. Single plane of rotation:
- The single disturbing mass rotates in a specific plane.
- To balance the system, the balance masses need to be introduced in a plane parallel to this plane of rotation.
- This ensures that the forces generated by the balance masses act in the same plane, effectively canceling out the unbalanced force.

4. Minimum number of balance masses:
- To completely balance the system, two balance masses are required.
- One balance mass is placed on each side of the disturbing mass, symmetrically.
- These two balance masses create equal and opposite forces to counteract the unbalanced force generated by the disturbing mass.

In summary, when balancing a single disturbing mass, two balance masses are required to be introduced in a plane parallel to the plane of rotation of the disturbing mass. These balance masses create equal and opposite forces to counteract the unbalanced force and achieve balance.