The correct answer is waterways.

• Waterways are the cheapest mode of service.
  • The amount of loading and unloading goods is much cheaper if it has to travel a long distance.
• Inland waterways
  • ​Navigable rivers and lakes are used as inland waterways.
  • The Great Lakes in North America, the Ganga – Brahmaputra river system and the river Nile in Africa are some of the important inland waterways.
• Seaways
  • ​These are the oceanic routes and used for transporting goods from one country to another.
  • These routes are connected with the ports.
  • Some of the important ports in Asia are Mumbai and Singapore, New York, Los Angeles in North America, Rio de Jenario in South America, and Sydney in Australia.

• Roadways are the most commonly used means of transport, especially for short-distance travel.
  • Roads may be metalled or unmetalled.
  • There is a dense network of roads in the plains.
  • Roads can be built in deserts, high mountains, and forests too.
  • Roads built underground are called subways or under paths.
• Railways are economical and faster.
  • Steam engines have been replaced by diesel and electrical engines. 
  • Railway network is well developed in plain areas.
  • It is present in hilly terrain too, but not as much as in the plain areas.
• Airways 
  • ​It is the most expensive mode due to the high cost of fuel.
  • It is vulnerable to bad weather like fog and storms.
  • It is the only mode to reach even the most distant and remote locations which do not have a road/rail network.