"Unity of Command" means.

The unity of command principle is simple yet so crucial for the success of an organization. 

For the organization able to react quickly to changing situations, it has to have a clear command structure that allows orders from the top to be passed down through all levels of the organization.

If an employee receives orders from multiple superiors, they will likely not know how to respond in a given situation, or they might not even receive a clear order at all. 

This lack of clarity will lead to mistakes and chaos in the organization.

Final answer: Hence, the correct option is B. An employee shall receive orders from one senior only.