10GBASE-LR is the best choice for connecting two data centers that are 6.2 miles (10 km) apart via fiber, because it supports a maximum distance of 6.2 miles (10 km) over single-mode fiber. 10GBASE-SR and 1000BASE-SX are designed for short-range connections over multi-mode fiber, and they can only reach up to 1,312 feet (400 m) and 1,804 feet (550 m), respectively. 10000BASE-LX is a typo and does not exist as a standard. Reference: Network Transceivers - CompTIA Network+ N10-007 - 2.11 CompTIA Network+ Certification Exam Objectives2