Step 1: Sentence Comprehension.
The sentence implies Sara's decision was rapid and potentially impulsive. The term "impetuous," defined as acting quickly without forethought, aligns with this context.
Step 2: Option Analysis.
- "Impervious" signifies being unaffected, a poor fit for rapid decision-making.
- "Inarticulate" relates to unclear expression, irrelevant to decision-making processes.
- "Inerrant" denotes freedom from error, not applicable here.
Step 3: Determination. "Impetuous" accurately characterizes hasty or rash decision-making. Therefore, the correct selection is 2. impetuous.
Final Determination: \[\boxed{\text{The correct answer is 2. impetuous.}}\]