The correct answer is option 'C', which states that the world's total motorable road length is 15 million km. Here's an explanation of this answer:

Motorable roads are essential for transportation and connectivity, allowing people and goods to move efficiently from one place to another. The total length of motorable roads in the world is a significant measure of infrastructure development and connectivity.

To understand the world's total motorable road length, we need to consider the road networks across different countries and continents. While it's challenging to provide an exact figure, estimates can be made based on available data and studies conducted by various organizations.

Factors Affecting Total Motorable Road Length:
1. Country Size: Larger countries tend to have longer road networks due to their expansive territories.
2. Population Density: Higher population density often leads to the construction of more roads to accommodate transportation needs.
3. Economic Development: Developed countries generally have extensive road networks compared to developing nations.

Total Motorable Road Length:
1. Available Data: The World Bank, International Road Federation (IRF), and national transportation departments collect data on road networks. However, the data may not always be up to date or comprehensive for all countries.
2. Estimates: Based on available data, it is estimated that the total motorable road length in the world is around 15 million km. This estimate considers the road networks of all countries combined.

Importance of Motorable Roads:
1. Economic Growth: Motorable roads facilitate trade and commerce by connecting industries, markets, and consumers.
2. Social Connectivity: They enable people to access education, healthcare, and other essential services.
3. Tourism: Well-connected road networks promote tourism by providing access to popular destinations.
4. Transportation Efficiency: Efficient road networks reduce travel time, congestion, and transportation costs.

Conclusion:
The total motorable road length in the world is estimated to be approximately 15 million km. This figure represents the vast network of roads that facilitate transportation, economic growth, and social connectivity globally. It is important for countries to continue investing in infrastructure development to enhance their road networks and promote overall development.