That was nicely done!

you can't have a group of trees and expect "Pokemon professor" to be the given answer.

I have to agree with SonJonath.

you should have to accept trees as all four your pokemon one are types of tree. only connect rules would suggest this is correct

Folks are correct to point out the "Pokemon professors" group is also simply "trees." One category's existence doesn't preclude another's, especially when the other category preexists and is thus better known. Trees are older than Pokemon, in case you didn't know.