Question

When platelets attach to collagen threads in an injured vessel the platelets swell, form extensions and then stick together. as a result of these steps what structure is formed?

Answer 1

Answer is: platelet plug
 
When platelets stick to each other and to the blood vessel they form a plug in the hole of the vessel. This process is called platelet aggregation.

Before the plug forms,  a platelet will stick to the blood vessel (adhesion), change its shape and signal other platelets to come help (activation and secretion). Then they stick to each other (aggregation).