What element of fibrous joints gives the joint its ability to resist stretching and control the amount of movement at the joint?

Fibrous joints are the immovable joints which do not move so they are also known as fixed joints.  

These joints are connected by the connective tissue made up of collagen protein. The length of these connective tissue fibres controls the movement of the bones like:

1. Suture joints have short fibres so they are mostly immobile.

2. Syndesmosis present between tibia and fibula in the ankle have slightly longer fibers so they can show movement if needed.

Thus, correct answer is collagen and length of these fibers.

a, i am pretty sure.