What is the auto-MDIX feature on a switch?

What is the auto-MDIX feature on a switch?
the automatic configuration of full-duplex operation over a single Ethernet copper or optical cable
the automatic configuration of an interface for a straight-through or a crossover Ethernet cable connection
the ability to turn a switch interface on or off accordingly if an active connection is detected
the automatic configuration of an interface for 10/100/1000 Mb/s operation

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