"They are walking."
1) chodzę is in first person singular, you would need to write "one chodzą"
2) while this sentence is so without context, "one chodzą" could be a good translation, the course policy is to remind learners that "chodzą" is for repeated action or action without any direction, or having the ability to walk, and idą is for action that takes place right now and has a direction.
basically this course usually makes you write chodzę for I go/walk and idę for I am going/walking.
I cannot find this sentence in the Incubator, so it seems to have been deleted. And sometimes deleted sentences show up to learners anyway, because... well, we don't know.
Yes, "jadą" shouldn't be accepted here. Perhaps this sentence used to be "They are going" at some point and then because of some changes something got wrong and it was deleted.
Verbs of Motions are actually this 1% of verbs that do show the difference between Present Simple and Present Continuous. Generally, "iść" is used for Present Continuous (it happens right now, or is used in the future meaning) and "chodzić" is used for Present Simple.
However, the problem of the sentences in this course is that many of them have no context at all. And if someone "is walking" right now, but without any direction, any goal (just walking around the park), that can also be "chodzić". But it doesn't walk for "going".
iść = to be going (on foot), to be walking
chodzić = to go (on foot), to walk, to be walking 'around'