What role does the F$-$function play in the Feistel structure of DES?

Question $5$ options:

It generates the $56-$bit DES key from the original $64-$bit key.

It encrypts data using a public key infrastructure system.

It expands, substitutes, and permutes the right half of the data block using the round subkey.

It directly swaps the left and right halves of the data block without any further processing.