visibility of the opposition's playing cards.
there's no need to put "card(s)" because it is a conditional sentence, the opposition may have any number of cards or none at all, but all of their cards will be visible if the setting is selected, therefore it is cards. this is intuitive from the context.
The point of the phrase is asking if their action of playing a card should be visible, not if their playing cards should be visible.
No, I thought you could see their cards when this setting is selected? If you go to the cards window there is a drop-down menu where you can select whose cards you want to see?
Edited 9/10/2014 08:10:14