Answer :mono-alphabetic
Mono-alphabetic Substitution Ciphers are probably the most common form of cipher. They work by replacing each letter of the plain-text with another letter (or possibly even a random symbol).
A mono-alphabetic substitution cipher, also known as a simple substitution cipher, relies on a fixed replacement structure. That is, the substitution is fixed for each letter of the alphabet. Thus, if "a" is encrypted to "R", then every time we see the letter "a" in the plain-text, we replace it with the letter "R" in the cipher-text.
Reference : Example is here