The correct answer is D) Contamination of the cargo. When loading dissimilar products through the same piping system aboard a tanker, the primary consideration is to avoid contamination of the cargo. The order of loading must be carefully planned to prevent cross-contamination between different cargoes, as this can affect the quality and safety of the products. Factors such as Reid vapor pressures, flash points, and specific gravities are important, but they are secondary to the need to maintain cargo integrity and purity. The other options, while relevant in other contexts, are not the primary factor in determining the order of loading for dissimilar products through the same piping system. Contamination of the cargo is the key concern that must be addressed to ensure safe and effective cargo operations aboard a tanker.