The verified answer is D. Bilingual Evaluation Understudy (BLEU) score. The company wants to evaluate whether the chatbot’s generated language matches a target communication style that includes creative spelling and shortened words. AWS AI Practitioner guidance lists BLEU, ROUGE, and BERTScore as relevant metrics for assessing foundation model performance. AWS Prescriptive Guidance describes BLEU as a metric-based evaluation method that measures n-gram overlap and focuses on precision. Because the requirement is about matching a particular style of generated language against reference examples, BLEU is the best fit among the given options.

F1 score is incorrect because it is primarily used for classification-style problems where precision and recall are combined into one score. It is not the correct metric for evaluating generated text style.

BERTScore is incorrect in this specific question because BERTScore focuses on semantic similarity using contextual embeddings. That can be useful when paraphrases should be accepted, but the question emphasizes surface language style: creative spelling, shortened words, and target-audience wording. Those are closer to token and phrase overlap against reference-style responses.

ROUGE is incorrect because ROUGE is commonly used for summarization and measures overlap with reference text, especially recall-oriented overlap. The scenario is not primarily asking whether the chatbot preserved all important content from a source document. It is asking whether the generated response matches a language style.

BLEU is therefore the best answer because it can compare the generated chatbot output to reference examples written in the desired teenage language style and measure how closely the model’s wording overlaps with those references.