Step 1: Identify the core ideas in the given passage.
The passage focuses on: (i) scientifically selecting employees based on their competencies, (ii) assigning work that suits each person's individual capabilities, and (iii) providing required training to increase efficiency and prosperity of workers.
Step 2: Understand the principle Science, not Rule of Thumb.
This principle asks managers to replace gut-feel and tradition with scientifically determined methods. It focuses on identifying the one best way to do a job, not on individual development.
Step 3: Understand the principle Harmony, Not Discord.
This principle promotes mental revolution and eliminates conflict between management and workers. It focuses on mutual cooperation and shared prosperity, but not specifically on individual selection and training.
Step 4: Understand the principle Cooperation, Not Individualism.
This principle encourages teamwork and collaboration between management and workers. It is about collective effort, not individual capacity-based selection.
Step 5: Understand the principle Development of Each and Every Person to His or Her Greatest Efficiency and Prosperity.
This principle directly advocates for scientifically selecting each worker, assigning them work that matches their physical and mental capacity, and continuously training them. The goal is to ensure every individual worker reaches their maximum potential and achieves prosperity alongside the organisation.
Step 6: Match and confirm.
The passage perfectly describes this fourth principle.
\[ \boxed{ \text{(D) Development of Each and Every Person to His or Her Greatest Efficiency and Prosperity.} } \]