Final answer:
Calcisand can expand in a reptile's GI tract and potentially cause rupture, while cricket water gel and mealworms do not pose the same risk.
Step-by-step explanation:
The substance among the provided options that can expand in a reptile's gastrointestinal (GI) tract and potentially cause rupture is b) Calcisand. When ingested, calcisand can clump together within the moist environment of the reptile's digestive system, leading to impaction and possibly life-threatening obstructions. Cricket water gel is designed to provide water to insects without the risk of them drowning, and it does not pose the same risk of expansion within the GI tract. Mealworms, while they can pose other health risks if fed inappropriately or excessively, such as a fat imbalance, do not expand to the extent that can cause GI rupture in reptiles.