The correct answer is option 3: initiating blood clotting. Platelets, also known as thrombocytes, play a crucial role in the process of blood clotting, which is necessary for preventing excessive bleeding. When there is an injury or damage to blood vessels, platelets are activated and stick together at the site of injury, forming a platelet plug. They release chemicals that attract more platelets to the area and promote the activation of other components involved in clot formation, such as fibrinogen and coagulation factors. The clot formation ultimately leads to the formation of a fibrin mesh that stabilizes the platelet plug and forms a clot. This process is essential in sealing wounds and preventing further blood loss.

Options 1 and 2, transporting oxygen and carbon dioxide, are functions primarily performed by red blood cells, not platelets. Red blood cells contain a protein called hemoglobin, which is responsible for carrying oxygen from the lungs to the tissues and transporting carbon dioxide back to the lungs for removal.

Option 4, initiating degradation of urea, is unrelated to the function of platelets. Urea is a waste product of protein metabolism that is primarily metabolized and excreted by the kidneys, not platelets.