The organ that excretes water, fat, and various catabolic (breakdown) waste products is the kidney. The kidney is an essential organ of the urinary system and plays a vital role in maintaining our body`s fluid balance, filtering waste products from the blood, and producing urine. It helps regulate the concentration of water and electrolytes in the body and removes metabolic waste products like urea and excess salts. The kidneys also filter out any excess water and fat-soluble waste products.

Option 2, the skin, is responsible for excreting sweat, which contains water, salt, and some waste products, but it does not excrete fat or catabolic wastes.

Option 3, the spleen, is part of the immune system and is not involved in waste excretion.

Option 4, the salivary glands, produce saliva, which aids in the digestion process but does not excrete water, fat, or catabolic wastes.