I think it is a rhetorical question directed at individuals. The WTS generally quotes these verses out of context, ignoring the verses immediately preceding:
But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into.
So you also must be ready, because the Son of Man will come at an hour when you do not expect him.
In that context, when Jesus asks, "Who then is the faithful and discreet slave?", He is obviously alluding to what He has just said in the above verses. Each individual must decide for himself whether he will be "faithful and discreet," faithfully carrying out his Master's assignments, when the Master returns unannounced at a time that the servant would not know. The alternative would be to take the position of the evil slave - "my master is delaying" - and carry on as if the master would never return.
Interestingly, it is only one who expects the master to return at a specific time who can really conclude that the master is delaying - given the WTS's history of date-setting, they would seem to qualify better as the evil slave than the faithful one.
And if the WTS (or anyone else) really believes that the FDS is a "class" that was appointed by Jesus to a position of authority, the burden of proof for such a position is clearly upon them, since the plain reading of the text does not even come close to supporting such a claim.