Final answer:
Cells combine to form tissues, multiple tissues form an organ, several organs form an organ system, and organ systems operate together to create an organism.
Step-by-step explanation:
In multicellular organisms, the hierarchy of structure starts from cells which are the basic units of life. These cells combine to form tissues, which are groups of similar cells working together to carry out specific functions. Examples of tissues include muscle tissue and nervous tissue. Continuing in this hierarchical structure, several tissues combine to form an organ such as the stomach, heart, or brain. Furthermore, multiple organs work together as a system known as an organ system, for example, the digestive system and the circulatory system. Lastly, the combined effort of various organ systems establishes a living entity referred to as an organism, such as a human, animal, or plant.