Final answer:
Option 'd. STAC (default network theory)' is the model not discussed in the provided textbook excerpts, as the common descriptions of STAC do not align with 'default network theory'.
Step-by-step explanation:
The question asks about models that explain brain reorganization and compensation for age-related changes. Each option provided (P-FIT, HAROLD, CRUNCH, STAC) represents a theory or model that is related to cognitive aging or brain function. However, based on the context provided, we do not find a specific mention of a model named 'default network theory' in relation to STAC, or as a standalone model directly addressing the idea of brain reorganization and compensation for age-related changes provided in the textbooks referenced. The other theories (P-FIT, HAROLD, and CRUNCH) directly address brain function and compensation strategies as individuals age.
Based on the references provided, STAC (Scaffolding Theory of Aging and Cognition) is an existing theory on brain reorganization, but the description of it as 'default network theory' does not align with the common descriptions of STAC. Therefore, 'd. STAC (default network theory)' seems to be the model not discussed in the provided textbook excerpts.