A bone is considered an organ.
Explanation: Bones are rigid organs that form part of the endoskeleton of vertebrates. They are composed of connective tissue and contain bone cells, collagen, and minerals such as calcium. While bones are made up of different types of tissues, including bone tissue, marrow, cartilage, and connective tissue, they function as organs due to their specific roles in the body. Bones provide structure, protect organs, anchor muscles, and store calcium. Therefore, the correct answer is b) organ.