What is database sharding?

Is a method to increase the read throughput by replicating the data into multiple DB servers and offloading the reads from the primary instance.

Is a method of having multiple server working together, but managed by a single instance of a running database server.

Is a method of horizontally scaling an RDS database by using the same underlying shared storage for the DB$,$ but having multiple instance accessing the same storage for better throughput.

Is a method to increase the write performance of a DB by splitting the data into chunks and storing in different DB servers.