1000BASE-LX is a fiber optic Ethernet standard that supports distances up to 10 kilometers with single-mode fiber, and typically up to 550 meters using multimode fiber. It operates at a speed of 1 Gbps, which makes it the best solution for the given requirement. 1000BASE-T is designed for copper cabling and can only support distances up to 100 meters, which does not meet the long-distance requirement. Similarly, 100BASE-FX is a fiber standard but it is designed for shorter distances and operates at a lower speed of 100 Mbps. 10GBASE-LR is designed for distances up to 10 kilometers as well, but it supports speeds of 10 Gbps, which exceeds the specified requirement and may not be the most cost-effective option considering the company's request for 1 Gbps.