The best activity for modeling physical weathering of rock would be:
B. Place several cookies in a box, and then shake the box.
This activity simulates physical weathering because shaking the box represents the mechanical breakdown of rocks into smaller pieces through physical forces, similar to how rocks are weathered by physical processes such as abrasion, freeze-thaw cycles, or impact from other materials.
The other options involve chemical reactions (A), erosion via moving water (C), or melting (D), which do not accurately represent the concept of physical weathering.