The mode of transport that conveys goods at low cost per unit is

This question tests knowledge of the comparative advantages of different modes of transport used in commerce.

Rail transport is able to carry very large and bulky loads in a single trip using wagons linked together, which spreads the running cost of the journey over a much greater quantity of goods. This is what makes the cost per unit of goods carried by rail relatively low compared with the other common modes, especially over long distances and for heavy or bulky commodities.

Air transport is the fastest mode available, but it is also the most expensive because of high fuel consumption and limited cargo capacity relative to cost, making its cost per unit high rather than low. Road transport offers flexibility and door-to-door delivery, but vehicles can only carry comparatively small loads at a time, so the cost per unit tends to be higher than rail for large-scale movement of goods, especially over long distances. Pipeline transport is very efficient, but it is restricted to liquids and gases, such as crude oil or water, and cannot be used to convey ordinary manufactured or general goods at all.

When a question asks which mode of transport is generally cheapest per unit for moving large quantities of goods, rail is the standard answer because of the economies of scale that come from moving many wagon-loads together.