The correct answer is: D. Unity of command

Neo-classical organization theory emphasizes on unity of command, which is the principle that each employee should have only one supervisor. This principle is based on the idea that it is more efficient and effective for employees to have a clear line of authority and to know who their supervisor is.

A flat structure of organization is one in which there are few levels of hierarchy. This type of structure can be more efficient and effective because it allows for faster communication and decision-making. However, it can also be more difficult to coordinate activities and to ensure that employees are working towards the same goals.

A tall structure of organization is one in which there are many levels of hierarchy. This type of structure can be more effective in large organizations with complex tasks. However, it can also be more difficult to communicate and make decisions, and it can be more expensive to operate.

Co-ordination by hierarchy is the process of ensuring that the activities of different parts of an organization are aligned with each other. This can be done through a variety of methods, such as setting goals, developing plans, and monitoring performance.

In conclusion, neo-classical organization theory emphasizes on unity of command. This principle is based on the idea that it is more efficient and effective for employees to have a clear line of authority and to know who their supervisor is.