I know the answer of this qustion :

What RC4 key value will leave S unchanged during initialization? That is, after the initial permutation of S, the entries of S will be equal to the values from 0 through 255 in ascending order.

is :

Use a key of length 255 bytes. The first two bytes are zero; that is K[0] = K[1] = 0. Thereafter, we have: K[2] = 255, K[3] = 254, . . . , K[255] = 2.

but I want someone clarify this answer please.