The correct answer is C) Framework, primarily the corner posts. The framework, especially the corner posts, provides the majority of the structural strength to an aluminum-sided container. The corner posts act as the primary load-bearing members, transferring the weight of the container and its contents to the supporting structure. The aluminum side and end panels, while important for enclosure and protection, do not contribute significantly to the overall structural integrity of the container. The corner fittings and the framework and siding acting as a girder are secondary factors in the container's strength.