The correct answer is (d), the United Kingdom. The Channel Tunnel, also known as the Eurotunnel, is a 50.5-kilometre (31.4 mi) undersea rail tunnel connecting England and France beneath the English Channel. The tunnel is the longest underwater tunnel in the world, as well as the longest undersea tunnel to connect two land masses. It was opened on 9 December 1994 and is operated by Eurotunnel, a private company. The tunnel carries passenger and freight trains between London and Paris, and between London and Brussels.
Option (a), France, is incorrect because the Channel Tunnel does not connect France to any other country.
Option (b), Turkey, is incorrect because there is no intercontinental under-sea railway in Turkey.
Option (c), Russia, is incorrect because there is no intercontinental under-sea railway in Russia.