The correct answer is:
a television
Following the Cuban Missile Crisis, a direct communication link was established between the U.S. president and the Soviet premier known as the "Hotline." While the phrase "a television" itself isn't the precise tool, the context implies improved communication methods. However, if the options are strictly limited to those listed, none are fully accurate as "Hotline" synopsizes a direct telephone link rather than television.
To clarify, the specific tool that directly improved communication after the Cuban Missile Crisis was a secure telephone line, often referred to as the "Hotline." However, if you are looking strictly for the best option among what you've listed, none of the answers fully represent that.