Which significant improvement made instant message transmission possible across the seas in 1866?

The significant improvement that made instant message transmission possible across the seas in 1866 was the transatlantic cable. This technological advancement allowed for the first successful telegraphic communication across the Atlantic Ocean, linking North America and Europe. Prior to the installation of the transatlantic cable, communication between the continents was slow and often relied on ships carrying messages. The cable enabled real-time communication, transforming international relations and trade.

The transatlantic cable encompassed a series of underwater telegraph cables that transmitted electrical signals over long distances, making it a groundbreaking form of long-distance communication. This innovation marked a pivotal moment in history, as it foreshadowed the global interconnectedness that would grow in the following decades.

Other options, such as wireless radio, television broadcast, and satellite communication, came much later in history and were not technologically feasible or developed during that time period. Wireless radio, for example, emerged in the early 20th century, and television and satellites did not appear until the mid-20th century. Thus, the transatlantic cable stands out as the critical invention that enabled instant messaging across oceans in 1866.