The term that would best represent X would be tissue.
The simplest level of organization of living things is the cell followed by tissue, organ, organ system, and then the organism itself.
- The cell represents the simplest unit of existence
- Cells come together form tissues in order to perform a function
- Different tissues come together to function as a unit known as organs
- Different organs make up the system and the body of an organism is made up of different systems.
More on levels of organization of living things can be found here: https://brainly.com/question/282455?referrer=searchResults