"They come to realize that their bag fell."
Translation:Mereka menyadari bahwa tas mereka jatuh.
They come to realize that their bag fell.
Meaning: Mereka menyadari kalau tas mereka jatuh.
Mereka menyadari bahwa tas mereka jatuh.= not accepted, reported
I think 'kalau' cannot be used here to translate this English sentence.
'kalau' = when/if
It would give the sentence a different meaning.
It looks as if this has been changed to "bahwa" anyway, at least in the options presented (if not at the sentence at the top of this page) - I had it as a "Mark the Correct Meaning" question (6/19). But one of the better Memrise courses (curated by guestgulkan), based on an Indonesian course at the University of Queensland, has this for a later "kalau" entry:
that - when used as a conjunction the same as bahwa. It is commonly spoken more than "bahwa"
and if you look at what I think is an excellent resource, sealang.net, the definition of kalau includes (admittedly way down the entry!):
8 that; see: bahwa I.
It notes that this sense is derived from Javanese. (It gives a large number of sentences using "kalau", and a few of these do support "that" in English as a translation; but most seem to be the more common "if" meaning.)