The correct option is Saliva.
Explanation
Excretion is the process of removing metabolic waste products from the body. While the kidneys function as the primary excretory organs, accessory excretory organs such as the lungs, liver, skin, and salivary glands also contribute to the elimination of specific waste substances.
Option Analysis
- Saliva (Correct): Salivary glands primarily secrete saliva to aid in digestion and lubrication. However, they also facilitate the elimination of small amounts of nitrogenous wastes, particularly urea, from the body.
- Cerebrospinal fluid (Incorrect): This fluid surrounds the brain and spinal cord, providing mechanical protection and nutrient transport. While it removes metabolic waste from the central nervous system into the blood, it is not a secretion used to eliminate waste from the body to the external environment.
- Synovial fluid (Incorrect): This is a viscous fluid located within synovial joint cavities. Its function is to lubricate joints and nourish articular cartilage, not to excrete waste products.
- Aqueous humour (Incorrect): This fluid maintains intraocular pressure and nourishes the eye's lens and cornea. It does not function as a route for the systemic elimination of nitrogenous waste.
Key Takeaway: In addition to urine, small quantities of nitrogenous wastes (such as urea) are eliminated through sweat and saliva.