I came across this question, and found the same meaning in the dictionary for both options
(a-危ない) or (b-危うい) ですから 下がってください
however the answer booklet says it is (a-危ない), what is the difference?
They have the same meaning of "dangerous", but 危うい is used more in the written language (文語), whereas 危ない is used more in the spoken language (口語).
Here's a Chiebukuro question asking about this.
大辞林 has a note in the entry for 危ない
(Warning: possibly inaccurate translation ahead). Basically it says that 危ない has replaced 危うい in modern times. Nowadays, the usage of 危うい is literary-like, and expresses the urgency/imminence of a worst case scenario like death or decimation. In contrast with that, 危ない expresses that something bad is going to happen, regardless of whether it involves bodily harm (threat to life) or not.
Additionally, in 語源's entry on 危ない, it mentions that
The 元々 seems to suggest that it is not the case now though, hmm...