Final answer:
Uric acid is the nitrogen waste product that is the least soluble in water, forming a white paste or powder when excreted by birds, reptiles, and terrestrial arthropods.
Step-by-step explanation:
The nitrogen waste product that is the least soluble in water is uric acid. In contrast to ammonia and urea, uric acid is not readily soluble in water and tends to form a white paste or powder. While ammonia is highly soluble and urea has moderate solubility, uric acid's low solubility makes it particularly advantageous for animals in arid environments as it conserves water when excreted. Birds, reptiles, and most terrestrial arthropods excrete uric acid, which is a product of ammonia metabolism more complex and energy-intensive than conversion into urea or ammonia. Nitrogenous wastes like ammonia and urea, although more soluble, are toxic and therefore are efficiently converted and excreted in different forms depending on the organism and its environment.