To illustrate the formation of blood cells, the most useful model among the given options would be a computer-generated model.
This is because a computer-generated model can provide dynamic visualizations of the complex processes involved in blood cell formation (hematopoiesis), including the differentiation of stem cells in the bone marrow. It can represent various cell types, their development stages, and the interactions within the microenvironment, making it easier to understand the biological processes involved.
In contrast, a dissected pig or a plastic bone would provide physical representations but might not effectively demonstrate the physiological processes. A photograph of a bone would be static and less informative regarding the dynamic process of blood cell formation.