Which type of cable do you use to connect two Cisco Nexus switches that are 50 km apart?

MMF (Multi-Mode Fiber) suffers from modal dispersion that limits it to roughly 550 meters at 1 Gbps and even shorter runs at higher speeds - far short of 50 km.

TSMF (representing tunable or extended-reach single-mode fiber technology) uses a narrow-core fiber with a coherent laser source that can sustain signal integrity over distances of 40-80 km or more, making it the only listed medium suitable for a 50 km inter-switch link.

UTP (Unshielded Twisted Pair) is a copper medium limited to 100 meters by signal attenuation, making it completely unusable for a 50 km connection.

STP (Shielded Twisted Pair) shares the same 100-meter copper distance limitation as UTP; shielding addresses electromagnetic interference but does not extend transmission range.