What technology was unveiled by the US in the 1980s that allowed aircraft to be virtually invisible to radar?

The concept of stealth technology was a significant advancement in military aviation developed in the 1980s, allowing aircraft to be designed in a way that minimizes their radar signature. This technology employs various design principles, such as special shapes and surface materials that absorb radar waves or deflect them away from the radar source, making the aircraft effectively "invisible" to enemy radar systems.

Stealth aircraft, like the F-117 Nighthawk and later models such as the B-2 Spirit, utilized these principles to enhance operational effectiveness and survivability in hostile environments. The introduction of this technology transformed air combat strategies and allowed for missions to be conducted with a higher probability of success, as enemy radar detection was significantly reduced.

While the other options mentioned—drone technology, electronic countermeasures, and advanced missile systems—involve critical advancements in military capabilities, none specifically address the groundbreaking nature of stealth technology in rendering aircraft less detectable and significantly changing the dynamics of aerial warfare.