Final answer:
The best choice to replace 'thin' in the sentence for a positive connotation is 'slender' because it suggests a gracefully thin figure that is complementary to the context of elegance.
Step-by-step explanation:
The sentence 'The model was tall, ____, and elegant as she presented Ricardo's latest designs at the fashion show' requires a word that replaces 'thin' and retains a positive connotation. The word 'slender' is the best choice as it suggests a gracefully thin figure, which is positive and suitable for describing a model. The other options such as 'skinny,' 'scrawny,' and 'skeletal' have more negative connotations, implying an unhealthy or excessively thin appearance which is not ideal in the given context of elegance and fashion.