Let Ek$($x$)$ be the block encryption function of a block cipher. In particular, E is the block encryption algorithm, k is the symmetric key, and x is a block$-$length input.

The plaintext message is chopped into blocks m$0,$ m$1,$ m$2,...$ where mi is the ith plaintext block.

Also, we have a pseudo$-$random number generator G$($i$,$seed$)$ that takes a seed $($some initialisation number$)$ and for each index i generates a block$-$length pseudo$-$random output.

Suppose Alice uses the following algorithm to generate her ciphertext blocks:

ci $=$ Ek$($mi$)$ XOR Ek$($G$($i$,$ seed$))$

Which one is the correct decryption algorithm by Bob $($Alice and Bob have agreed on E$,$ G$,$ k$,$ seed beforehand, and D is the block decryption algorithm corresponding to E$.$

a$.$

Dk$($ci$)$ XOR G$($i$,$ seed$)$

b$.$

Dk$($ci$)$ XOR Ek$($G$($i$,$ seed$))$

c$.$

Dk$[$ci XOR Ek$($G$($i$,$ seed$))]$

d$.$

Ek$($G$($i$,$ seed$))$ XOR ci

e$.$

Dk$[$ci XOR Dk$($G$($i$,$ seed$))]$