Explanation:

- Observation: Managers develop principles by closely watching and analyzing how things work in real-world settings. They observe patterns and outcomes to understand effective practices.

- Experimentation: Management principles are also born from testing various methods and strategies. Managers set up experiments to see what works best, applying scientific methods to manage better.

- Personal experiences of the manager: Managers learn from their own experiences over the years. These experiences shape how they handle challenges and decisions, refining management principles.

- All of the above: This option is correct. Management principles are universally developed through a blend of these processes: observation, experimentation, and personal experiences.

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