In a block cipher, diffusion can be accomplished through:

A.
Nonlinear S-boxes

B.
Substitution

C.
Permutation

D.
XORing

Explanation:
Diffusion is aimed at obscuring redundancy in the plaintext by
spreading the effect of the transformation over the ciphertext. Permutation
is also known as transposition and operates by rearranging the
letters of the plaintext.
*Answer substitution is used to implement
confusion in a block cipher. Confusion tries to hide the relationship
between the plaintext and the ciphertext. The Caesar cipher is an
example of a substitution cipher.
* XORing is incorrect since XORing, for example, as used in a stream cipher, implements
confusion and not diffusion. Similarly, nonlinear S-boxes implement substitution. In DES, for
example, there are eight different S-boxes that each has an input of 6 bits and an output of 4 bits.
Thus, nonlinear substitution is effected.