The correct option is A Collagen
Proteins which are involved in providing structural support to the cell are called structural proteins. For example - collagen is the protein which is present in the fibrous tissues such as tendon, ligament and skin.
Enzymes are proteins that catalyze biochemical reactions.
Antibody is a type of defence protein that is produced by the immune system and helps in removing them from the body.
Casein is a nutritional protein found in milk.