Apolyalphabetic cipher uses many
a) keys
b) transpositions
c) codes
d) monoalphabetic substitution rules

d) Mono-alphabetic substitution rules

Explanation: Poly-alphabetic cipher is the cryptography that is done by using the many alphabets in substitutional way. They are supposed to have a good security because they use cipher text alphabets for the encryption and then decrypts it.

Substitution of the alphabets is done by the rule of the mono-alphabetic substitution rules.Other given options are incorrect because they are not used in the cryptography of the alphabetic cipher. Thus option (d) is the correct answer.