Because they contain ________, drinks like colas, coffee, and tea should be avoided as rehabilitation fluid-replacement drinks?

The correct answer is caffeine. Caffeine, found in beverages such as colas, coffee, and tea, has diuretic properties which can lead to increased urination. This effect can potentially lead to dehydration, undermining the goals of rehabilitation and fluid replacement, which aim to replenish lost fluids and rehydrate the body.

When looking for effective rehabilitation drinks, it is essential to choose options that enhance hydration and replenish electrolytes without the adverse effects associated with caffeine. Options that are designed for recovery typically contain balanced electrolytes and are non-caffeinated to ensure effective hydration. Other substances like aspartame, nicotine, and tannin do not play the same role regarding hydration and fluid replacement, making them less relevant in the context of the question.