Oops! Now that you say that I see both possibilities can happen. I guess it all depends on definitions: are we to understand that the vertices are moved (together with their tag = letter) or else the vertices are kept fixed and we move only the letters??
In the first case the right option is correct, since vertex A is moved to position prviously occupied by vertex B, but in the second case the left one is the correct one.
It's not something to get hung up on - the group or whatever it gets called is generally used only for training purposes, as are the diagrams that illustrate the permutations. So whichever convention is being used, you'll be off them quicker than training wheels on your first pushbike.