In a typical Fiestel round, what happens to the left and right halves of the data block?

Select one:

a$.$ The right half is encrypted using a subkey, and the result is XORed with the left half.

b$.$ They are swapped back and forth, enhancing confusion and diffusion.

c$.$ Both halves are independently subjected to complex mathematical operations.

d$.$ The left half is used as input for the next round, and the right half is discarded.