Concept:Water transport is the most economical and efficient choice for moving heavy or bulky goods between countries over long distances.
Explanation:Air transport is extremely fast but its cost is very high, making it unsuitable for large or heavy cargo.
Land transport becomes expensive and impractical when carrying very heavy or bulky goods across long international routes.
Pipeline transport is restricted to specific products like crude oil, natural gas, or other liquids, so it cannot carry general bulky goods.
Water transport, particularly sea transport, offers huge vessels that can carry enormous quantities of cargo in a single trip.
The operating cost per tonne of cargo is comparatively lower than any other mode of transport.
Additionally, waterways require little maintenance and use natural routes, which further reduces the cost.
Hence, for bulky goods moved across countries, water transport is undoubtedly the cheapest mode.
Answer:D. Water