This solution is brought to you thanks to the website Gamebanshee about Van Helsing.
If you prefer a neat video over pictures, check at the end of this guide ( courtesy of Zasta 360GameTV)
There is 4 wisps located in 4 different zones of the game.
First one is in Croakwood, it's "named" the Nameless wisp ( for now...)
Location :
http://hpics.li/bc88606You have 3 choices to answer the riddle correctly :
You can say "I didn't take your name" and then allow Katerina to name it Hearthfire; you can say "Begone, evil spirit" and then name it Hearthfire yourself; or you can say "Begone, evil spirit" and then name it Lifespark. Any other responses will cause the wisp to attack you.
The second wisp is in DreadworksLocation :
http://hpics.li/bc9415bQ : It begins but has no end... yet it ends all that begins
The third wisp is in The Old TownLocation :
http://hpics.li/a03b640Q : What is the answer to Life, the Universe, and Everything?
The last wisp is in The Pocket RealmLocation :
http://hpics.li/a6e9f39Q : I touch the earth, I touch the sky. If I touch you, you'll surely die!