What you are asking for is a very complex system. It's possible, but it is very difficult. I wont go through how to make the entire thing, but you basically need to create a dialogue system (or borrow one from the Stencylforge) and make some mods to it. You would store dialogue into a list as text. You would also store special parameters into the text string that tells the system what to do. For instance, let's say that I am displaying four lines of dialogue at a time. On the first line I would have something like this:
2_Yes_No_acceptQuest_rejectQuest_Hey there adventurer! Have you got a moment?
I could really use your help with something of the utmost importance!
There we have two lines of dialogue. Then we'd need to split the text in line one into a separate list so that we can analyze it. We would split it at the _. Next, we'd check to see the item at list position 0, which would be the 2. This signifies that we will have 2 possible choices. That also tells the dialogue to check the next 4 parameters.
So, now that we know we have 2 choices, we move on to list object #1, which is Yes. This will generate a button on screen with the text Yes. We send to that Yes Button parameter #3 which tells it to trigger event acceptQuest, when pressed, in all behaviors for our NPC. We then create the next button, No, which triggers rejectQuest.
Finally, we have the normal line of text which our npc will say. So, the final result of this code would look something like this in the dialogue box:
Hey there adventurer! Have you got a moment?
I could really use your help with something of the utmost importance!
1. Yes
2. No
Then, when the player clicks on acceptQuest or rejectQuest, you have the dialogue skip to the required position in the dialogue list to display the appropriate response, or you use a separate list of dialogue for that.
Like I said, it is a very complicated system. If you don't understand a single thing that I said, you probably need more experience with Stencyl.