I am going to go with: Parasailing is where you're soaring through the air with a parachute while being towed by a motorboat.
The reason why it is wrong is because the word where makes it sound like you are describing a place and not an activity. The other sentences make sense because parasailing is being described as an activity. Let me show you: parasailing is a sport...when parasailing you soar..., parasailing is soaring... all of these show that it is an activity.
Hopefully this helped and good luck.