The best option that explains how the details in the text support the development of the central ideas is:
A. They explain the different ways the White Elephant game can be played and the legend about the king of Siam, from which the game is named.
This choice effectively captures both the explanation of the current tradition of the White Elephant gift exchange (including its rules and dynamics) and the historical background linked to the legend of the king of Siam, which gives the game its name and context.