Which criterion is included when proving that a product defect existed at the time of sale?

Proving that a product defect existed at the time of sale hinges significantly on demonstrating the product's condition upon leaving the seller's control. This criterion is critical because it establishes the baseline state of the product when it was transferred to the buyer. If a product is defective when sold, it is generally the responsibility of the seller to ensure that it meets safety standards and quality expectations at that point in time. This principle ties into the concepts of strict liability and negligence in product liability cases, as it directly addresses the manufacturer's or seller's duty to provide a safe product.

The other options do not directly relate to the objective assessment of a product's physical condition at the moment it was sold. The consumer's understanding does not factor into the inherent defectiveness of the product itself, nor does the marketing strategy, which focuses more on how the product is presented rather than its physical integrity. While consumer complaints can indicate problems with a product, they do not serve as definitive proof of the product's condition at the moment of sale. Therefore, focusing on the product's condition upon leaving the seller's control is essential for demonstrating the existence of a defect in product liability cases.