What is formed when different tissues work together?

When different types of tissues come together, they form an organ. Organs are specialized structures that perform specific functions vital to the organism's survival. For example, the heart is an organ made up of muscle tissue, connective tissue, and nervous tissue, all working together to pump blood throughout the body. This collaboration between various tissues allows for complex functions that single tissue types alone could not achieve.

In contrast, the other options involve different levels of biological organization. An ecosystem refers to interactions between communities of organisms and their physical environment, while a population pertains to a group of individuals of the same species living in a particular area. A community comprises different populations interacting in a specific environment. Thus, the correct answer highlights the specific relationship between tissues and organ formation, underscoring the hierarchical nature of biological structures.