Which of the following is NOT a core selling tool of Splunk?

Customer testimonials serve as evidence of how existing clients perceive and benefit from a product or service, but they do not function as a core selling tool in the same vein as the other options listed. Core selling tools are typically direct methods or frameworks that a sales engineer uses to articulate a product's value proposition, demonstrate efficacy, or differentiate it in the market.

The Value Stack is a structured framework that helps potential customers understand the comprehensive benefits and value hence it is indeed a critical component in selling. The Whiteboard is often used in sales interactions for real-time discussion and illustration, helping to communicate complex ideas clearly. Differentiators are essential for positioning the product against competitors and highlighting unique features that may persuade a customer.

While customer testimonials can support sales efforts and provide social proof, they do not serve as a direct tool to engage customers in the same proactive manner as the others do. Thus, the correct response identifies customer testimonials as not fitting into the core selling tools category for Splunk.