Her impetuous behavior was attributed to her unbringing.

"Impetuous" means acting quickly and without thought, impulsive, or rash.

Option A- Sluggish means slow-moving or lacking energy. This is not the exact meaning of impulsive and thoughtless decision-making.  

Option B - Swift means fast or quick, which is not the opposite of impetuous. An impetuous action is not just quick but also unthoughtful.

Option C- Rash means hasty and reckless, which is a synonym of impetuous, not the opposite.

Option D- Sensible means showing good judgment, careful thought, and wisdom, which is the opposite of impetuous (reckless and impulsive).

Therefore, Option D is the correct answer.