"They walk there."

Translation: Ils marchent là-bas.

[PeterCollings]

Why not "Ils y marchent"?

[Sitesurf]

That is possible, actually.

[dan77g]

Still got marked down for this answer though...

[sw5230]

la and la-bas are both correct
what does Bas stands for? why is there a '-' in between?

[Sitesurf]

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.

[haveuever]

Why not "Ils vont"?

[rlind001]

Why is promener not accepted?

[MichaelWit7]

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.