What best describes the shifting of each letter a fixed number of spaces to the left or right?

What best describes the shifting of each letter a fixed number of spaces to the left or right?
A . Single substitution
B. Multi substitution
C. XOR
D. Bit shifting

Answer: A

Explanation:

Single substitution

https://en.wikipedia.org/wiki/Substitution_cipher#Simple_substitution

Substitution of single letters separately―simple substitution―can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet. The cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively) or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet.