Reason- Your sentence has two parts to it.
Dreams are ____ in and of themselves
when combined with other data, they can tell us much about the
Notice how the two parts are linked with each other with "but". The first part of the sentence tells you about a specific trait of dreams, on it's own stand. The second part tells you- when you combine dreams with other data, you can get more(a lot of) information about the dreamer. Thus, for the first part of the sentence(for the blank), you're looking for a word that is the opposite of what is given in the second part of the sentence(hence the usage of the word 'but').
Since the second part of the sentence informs us that dreams when combined with other data are very informative, then what follows automatically is- dreams, on their own, are pretty much meaningless, or not informative, that is- inscrutable. Something that is inscrutable means that something is not readily interpreted or understood.
Look up the meaning of inscrutable here-
incapable of being investigated, analyzed, or scrutinized;
not easily understood; mysterious; unfathomable: an inscrutable smile.
incapable of being seen through physically; physically impenetrable: the inscrutable depths of the ocean
Once again, before finalizing your answer, you do need to look at the other option you have. If you considered "Disordered", it gives you the impression that dreams, on their own, are merely disordered or misaligned(this may also imply that they do have the necessary information one might need to interpret them, you would only need to order/align the data). However, the next part of the sentence doesn't fit well with this, in terms of "other data" used only to "order" or "align" dreams.
For sentences like these, you would need to weigh both options against each other, and see for yourself which one makes the best sense.
Also, from my personal opinion, a good way of dealing with questions like these(and the one you posted before), is to familiarize yourself with the English vocabulary as much as possible.