Contractile proteins are found in muscles.
Explanation:
Contractile proteins, like actin and myosin, are primarily responsible for muscle contraction. They are essential for movement within the body. Bones, blood, and cartilage do not have the same function of contraction, making muscles the correct answer.
Why other options are incorrect:
Bones:
Bones provide structural support and storage of minerals. While they contain cells, these cells are not primarily involved in contraction. Bones are made up of primarily collagen and calcium phosphate.
Blood:
Blood transports oxygen, nutrients, and waste products. It contains red blood cells, white blood cells, and platelets, but none of these are involved in contraction.
Cartilage:
Cartilage is a connective tissue that provides cushioning and support. It is composed of chondrocytes and extracellular matrix, which are not designed for contraction.