The correct answer is option 4: Platelets.

Platelets are small, colorless blood cells that play a crucial role in clot formation, known as hemostasis. When a blood vessel is injured, platelets rush to the site and form a temporary plug to prevent excessive bleeding. They bind together and release chemical signals that attract more platelets to the area.

While all the options are components of blood, red blood cells (option 1) mainly carry oxygen, white blood cells (option 2) are responsible for immune responses, and hemoglobin (option 3) is the protein in red blood cells that carries oxygen. None of these directly contribute to the formation of blood clots.

Therefore, platelets are the correct answer, as they play a crucial role in the formation of blood clots to help stop bleeding.