Why not "Ils y marchent"?
That is possible, actually.
Still got marked down for this answer though...
la and la-bas are both correct
what does Bas stands for? why is there a '-' in between?
It is kind fo "down" that is there to emphasize "là" which is a very small word.
Note that "ici-bas" means: on the Earth as opposed to Heaven.
Why not "Ils vont"?
Why is promener not accepted?
Promèner is transitive and means to walk something while marcher is intransitive and means just means to walk.
Je promène mon chien: I walk my dog.
Je marche là-bas: I walk.