Which one of the following is not a part of the expression?

A non$-$recursive algorithm for generating $2$n bit strings of length n that implements bit strings as arrays and does not use binary additions.

a$.$ Generate the binary reflexive Gray code of order $4.$

b$.$ Trace the following non$-$recursive algorithm to generate the binary reflexive Gray code of order $4.$ Start with the n$-$bit string of all $0$s$.$

c$.$ Produces a pseudocode for a recursive algorithm for generating all $2$n bit strings of length n$.$

d$.$ For i $=1,2,...,2$n$1,$ generate the ith bit string by flipping bit b in the previous bit string, where b is the position of the least significant $1$ in the binary representation of i$.$