The heart of DES encryption lies in:

Select one:

a$.$ A complex sequence of XOR operations with the encryption key.

b$.$ Substitution boxes $($S$-$boxes$)$ that conceal the patterns present in the plaintext.

c$.$ permutations that execute complex bit rearrangements, disrupting the order of the data.

d$.$ All of the above, working together in multiple rounds of manipulation.