Correct Answer: Poignant

Solution : The first option is correct.

Poignant is an adjective that describes something that evokes a deep sense of sadness or regret, often with a mixture of sympathy and emotional intensity.
Pungent refers to a strong, sharp smell or taste and does not convey the specific emotional aspect of sadness or regret.
Impactful generally refers to something that has a significant impact, but it does not specifically capture the emotional tone of sadness or regret.
Unaffecting means not causing an emotional response and is the opposite of what is described in the given group of words.

Therefore, the option that can be used as a one-word substitute for evoking a keen sense of sadness or regret is poignant.