Step 1: Understand the physical factors affecting regional development.
Natural resources are the primary physical factor that impacts the development of a region, as they provide the necessary raw materials for industry and agriculture.
Step 2: Analyze each option.
- Option (A): Natural resources such as minerals, water, and land are essential for economic growth and development.
- Option (B): Sex-ratio, while important for social factors, does not directly affect regional development.
- Option (C): School register ratio is related to education but does not directly impact the economic development of a region.
- Option (D): Employment opportunities are influenced by many factors, including natural resources.
Final Answer:
The correct answer is (A) Natural resources.