What communication method did the signal corps primarily use in the Civil War?

The correct choice, which is visual signals, reflects the primary method of communication used by the signal corps during the Civil War. The signal corps utilized a variety of visual signaling techniques, such as flags, torches, and semaphores, to relay messages between units over long distances. These methods were particularly important in the context of the Civil War, where rapid and effective communication could significantly influence the outcome of battles.

Visual signals allowed the corps to quickly transmit tactical information without the need for wired communication lines, which were not readily available at that time. The use of flags was especially common, as they could be seen from afar, even during the chaos of battle. This reliance on visual signals was crucial for coordinating troop movements and strategies in a time when other means of communication were limited or nonexistent.

The alternative methods listed in the question were not the primary communication methods used by the signal corps during this period. For instance, reliable radio technology was not developed until after the Civil War. Telepathy is not a scientifically supported method of communication. Written orders were indeed utilized, but they were not the primary form of immediate communication used on the battlefield; visual signals played a far more critical role in real-time tactical decision-making.