The player who uses a grappling hook chooses which balloon to grapple to (if there is a choice) after seeing what card is played. Only balloons that played Ballast cards or Super Motors can be grappled, and then only if no sleeping gas was played by a balloon one level higher.
The rules say
Hooks can be used to hold onto balloons with Super Motor or Ballast Cards, it is the player's choice. A Sleeping Gas will break a Grapple.