[bbsbvb83]

As I see it, Bob, this is another poorly worded question. A libero replacement, which takes place as the R1 extends his/her arm to beckon for the serve is not necessarily illegal by rule. We must assume that the libero replacement mentioned in option "C" is illegal for some other reason. The rule reference is 10-4-1f plus PENALTY 1. You are correct that options "A" and "B" also result in UD.

EDIT: I suppose you could interpret 10-4-1f to mean that the libero replacement must take place before the R1 even extends his/her arm for the serve, without sounding the whistle or completing the signal with a sweeping motion toward the opposite shoulder. I know too many R1s in my area who extend their arm and then leave it there for several seconds before completing the mechanic.