Final answer:
Caffeine is the substance among the given options that acts as a mild diuretic, working by promoting vasodilation in the nephron, which increases urine volume.B. Caffeine
Step-by-step explanation:
The substance among the options provided that is a mild diuretic is B. Caffeine. Diuretics are compounds that increase urine volume, which causes a decrease in water conservation by the body. Caffeine, found in drinks like coffee and tea, functions as a diuretic by promoting vasodilation in the nephron, leading to an increased Glomerular Filtration Rate (GFR). This effect can lead to higher urine output. Unlike caffeine, substances like iron, phenylalanine, and calcium do not have diuretic effects.