Blood clotting or coagulation is a process in which platelets and proteins present in the blood plasma combined act to prevent excessive blood loss from the site of injury.
The cytoplasmic platelets release cytoplasmic granules that contain serotonin, ADP, and thromboxane and platelet factor 4.
There are more content in these granules such as vWF that helps in activating additional platelets that ultimately plug the site of injury and stops bleeding.
Thus, the correct answer is - cytoplasmic granules.
There are so many examples for that in different areas, like TPBi experiment carried out in our lab recently.Here's one link: http://www.alfa-chemistry.com/tpbi-cas-192198-85-9-item-282866.htm
