What do different tissues combine to form?

Different tissues combine to form an organ. Tissues are groups of similar cells that work together to perform specific functions. When different types of tissues come together, they create an organ, which has a defined structure and function within the body. For example, the stomach is an organ made up of several types of tissues, including muscle tissue for movement, epithelial tissue for lining, and connective tissue to support the structure.

The other choices do not accurately reflect the combination of different tissues. While cells are the building blocks of tissues, they do not combine to form an organ directly. An organ system, on the other hand, consists of multiple organs working together to perform complex functions but is one step further along the hierarchy of biological organization than tissues. Thus, the correct answer highlights the immediate step where multiple tissues unite to form a distinct organ.