Option (B) U.S.A is correct. The provided text states that 'The most dense rail network is found in the highly industrialised and urbanised region of East Central U.S.A. and adjoining Canada.' While the text mentions Belgium has the highest density globally, it is not among the options. Among the provided choices, the U.S.A. is the only country explicitly associated with the 'most dense' network description in the text. In contrast, Russia is described as having a 'very dense' network only 'west of the Urals', and Canada's railways are described as being distributed over 'sparsely populated areas'. Brazil's density is mentioned only relative to South America.