DESX is a variant of DES in which:

A.
The output of DES is bitwise XORed with 64 bits of key material.

B.
Input plaintext is bitwise XORed with 64 bits of additional key
material before encryption with DES, and the output of DES is also
bitwise XORed with another 64 bits of key material.

C.
The input plaintext is encrypted X times with the DES algorithm
using different keys for each encryption.

D.
Input plaintext is bitwise XORed with 64 bits of additional key
material before encryption with DES.

Explanation:
DESX was developed by Ron Rivest to increase the resistance of
DES to brute force key search attacks; however, the resistance of
DESX to differential and linear attacks is equivalent to that of DES
with independent subkeys.