Her impetuous behaviour was attributed to her upbringing.

Answer: Option C

Impetuous : acting or done quickly and without thought or care.
Poised : having a composed and self-assured manner.
Rash : acting or done without careful consideration of the possible consequences; impetuous.
Cautious : (of a person) careful to avoid potential problems or dangers.
Sluggish : slow-moving or inactive.

Antonym of Impetuous is Cautious