awalcus got it, congrats
can you explain pls?
i mean you can potentially react a horse traders to the second urchin. Yes if you already put it aside at the first opportunity you can't again, but that is also true for Beggar as you discarted it. Tunnel on the other Hand can only be reacted to the first urchin as you can't discard a card again with the second play
Mechanically, all three of Horse Traders, Beggar and Tunnel seem like they should fit the riddle. However, you can rule out Tunnel in two ways. It technically does not react to the Urchin. It reacts to being discarded, which is an effect of the Urchin. Flavour-wise, it doesn't make sense for the story because a Tunnel is inanimate and thus cannot react in surprise.
Beggar is trickier to rule out. Mechanically, it gets discarded so it technically isn't around for the second Urchin. Horse Traders is still there, just in "set aside" land. Flavour-wise, an Urchin probably wouldn't be asking for anything from a Beggar anyway, and many Beggars don't have doors upon which to knock (some do, of course).
the thing with these riddles is that you never know what you can take word by word and what you can also interpret in a different way:
in this case i took the "react surprised and dont give him anything" as not discart the tunnel =>not gain gold =>not give the urchung a gold because the first urchin came unexpected, then when the second urchin comes it is expected and you want to discart tunnel to give him a gold but now you can't anymore because of already 4 cards hand.
Tunnel does not react directly to urchin but the riddle does not strictly imply a reaction to urchin, just a reaction after the urchin comesSo with different interpretations of words different cards qualify for being the correct answer.
This interpretation might be a bit far-fetched but once you found something that fits one interpretation and that interpretation also disqualifies all other cards you don't think further into it.