Cause a problem and be forced to fix it
waaaiiiit why would i be forced to fix it? i would open the dark portal just to see if my spell works, if the world goes to complete chaos because of that who cares? i just enslave kil'jaden and rule this world
rest is pretty simple:
learn about limitatios or where demons come from, for me thats more or less the same. I mean we know they are from the nether, and we know that we can only have 1 out at a time (for whatever reasons). So both questions are kinda connected
c) powaaaaaar :P and a good reason why we are weaker then mages when we were acutaly mages and got some extra demon energy
d) depends on the puzzle, sadly since the internet exists you can find your answer for every puzzle in 20sec by typing in the quest so i'd go with defeat monsters.