Answer: Supporting your reply with relevant facts and data.
Explanation:
The rumors as regards the low quality of the product passed to Kim may have or may not have supporting evidences.
It is therefore good for him to support his response with relevant facts and figures if the rumors lack such and it's more imperative to do this if the rumors is backed by facts.
Knowing Kim area of expertise, establishing credibility as an expert and paying attention to implied communication are not enough to convince Kim on the quality of the product product.