Why did the united states and the soviet union install a “hot line” telephone system?

The United States of America and the Soviet Union installed a "hot line" for the purpose of faster communication between the heads of states of the two countries. Before the "hot line" was implemented in June of the year 1963, any communication between the leaders of the two countries took a long time and this could result in starting of a nuclear war at any time. To avert such dangerous situations the leaders of the two countries came together to start this system. The importance of the system was felt by both countries during the Cuban Missile Crisis in the year 1962.

Im pretty sure the answer is d.) is more densely populated.