Hi. Just a small matter.

If i win a challenge with Viserion ,for example, which has a response to discard a location ("...choose and discard 1 location..") ,what if i am the only one with locations or the only one who meet the requierements of the response, i have to discard my one location?

Same thing when i win Dragon, i must kill one of my characters if the other players does not meet the conditions of the response?

No. Responses are *always* optional. You don't *have* to trigger them. The controller of the card is usually the one who gets to decide whether or not to trigger a Response (there are a few Responses that can be triggered by an opponent).

Compare this to cards like Varys - his ability is passive, and it *has to* resolve when the play restriction is met, i.e. after he comes out of Shadows.

Ok. thank you