Which term would best describe a situation where one acts without forethought?

The term that best describes acting without forethought is "impetuous." When someone is impetuous, they tend to make decisions quickly and often without considering the consequences, leading to impulsive behavior. This contrasts with qualities such as being prudent or careful, which imply thoughtfulness and caution in decision-making processes. Additionally, "lucid" refers to clarity of thought or expression, which does not relate to the absence of forethought. Therefore, "impetuous" captures the essence of acting rashly and without deliberation, making it the most appropriate choice in this context.