If she ______ that important decision, she wouldn't be in this difficult situation now.
Correct: A
This is a Mixed Conditional sentence (Type 3 in the 'if' clause, Type 2 in the main clause). It describes a past hypothetical condition ('if she hadn't made that decision') leading to a present result ('she wouldn't be in this situation now'). The past perfect tense is used in the 'if' clause for the past condition.