The correct option is (a) Shift cipher
Easiest explanation: Shift cipher is a mono-alphabetic cipher in which each letter of the plaintext is substituted by another letter to form the ciphertext. It is the simplest form of substitution cipher scheme wherein the concept is to replace each alphabet by another alphabet which is ‘shifted’ by some fixed number between 0 and 25. In this scheme, both sender and receiver agree on a secret shift number for shifting the alphabet and this number lies between 0 and 25 becomes the key to the encryption. Occasionally, Caesar cipher is used to describe the Shift cipher when the ‘shift of three’ is used.