Final answer:
Quality is the customer-critical success factor best measured by warranty expense as it reflects the frequency of product defects and the additional costs to maintain warranty promises.
Step-by-step explanation:
The customer critical success factor that is best measured by warranty expense is Quality. Warranty expenses are incurred by a company when its products fail to meet quality standards and require repair or replacement. These expenses are a direct indicator of the quality of products as they demonstrate how frequently products fail or do not perform as expected, leading to additional costs for the company to uphold its warranty promises. Therefore, a lower warranty expense is typically indicative of higher product quality and fewer defects or issues.