U having to be in 4/5/6 means that V must be in 1/2/3 because V has to come before Z and X and if V were in 4, then we wouldn't have enough room for Z, X, and U.
V must be in 1/2/3
Y must be in 1/2/3
Y having to be in 1/2/3 means that X has to go in 5/6 because V, Z, and W come before it and if X were in 4, then we wouldn't have enough room for V, Z, W, and Y.
X 5/6
The question tells us we are subbing a new rule for the rule that W and Z are before X.
(E) says X performs in either slot 5 or 6.
The problem with (E) is that if we substituted this rule in, we could have W and X swapping places in 5 and 6, whereas in the old world W had to come before X.
With (E), we could have a game board that showed
Y V Z U X W
This board wouldn't be possible with our original rules, so (E) is out.
(A) says only U can perform in a later slot than X.
This means V, Z, W, and Y have to go before X and U is the only piece that can go after.
This restores the W before X from our original rules. It also restores Z going before X. V was already before X, so that stays the same. And Y before X? Yes, that was true in the original rules because we know that Y had to go in 1/2/3 and X was in 5/6, so (A) is a perfect rule substitution. In our original rules, U, having the ability to go in 4/5/6 was the only piece that could go after X in 5/6.
Hope this helps. Let us know if you have any more questions!