The correct answer is D) Contamination of the cargo. When loading dissimilar products through the same piping system aboard a tanker, the primary factor that must be considered is the potential for contamination of the cargo. The order of loading is critical to prevent cross-contamination between incompatible products, which could result in chemical reactions, quality issues, or other safety concerns. The other options, such as specific gravities (A), flash points (B), and Reid vapor pressures (C), are important factors to consider when handling cargo, but they are not the primary concern when determining the loading order for dissimilar products through the same piping system. The risk of contamination is the key factor that must be addressed to ensure safe and effective cargo operations.