The correct answer is C. a tissue. Detailed Explanation:

1. Definition of Tissue: In biology, a tissue is defined as a group of similar cells that work together to perform a specific function. Tissues are one of the fundamental levels of organization in multicellular organisms, which also include cells, organs, and systems.
2. Understanding the Hierarchy:
3. Cells: The basic unit of life. Each cell can perform various functions independently, but they often work together in groups.
4. Tissues: When similar cells come together to perform a common function, they form a tissue. For example, muscle tissue is made up of muscle cells that contract to facilitate movement.
5. Organs: Organs are structures made up of different types of tissues that work together to perform complex functions. For instance, the heart is an organ composed of muscle tissue, connective tissue, and nervous tissue.
6. Systems: Organ systems are groups of organs that work together to perform complex functions necessary for the survival of the organism. For example, the circulatory system includes the heart, blood vessels, and blood.
7. Why Option C is Correct:
8. The question specifically asks for a group of similar cells performing the same function. This perfectly describes a tissue, as it is defined by the similarity of its cells and their collective function.

Why the Other Options are Incorrect:

• A. an organ: An organ is made up of multiple types of tissues working together to perform a specific function. For example, the stomach is an organ that includes muscle tissue, epithelial tissue, and connective tissue. Therefore, it does not fit the definition of a group of similar cells.
• B. a system: A system is a higher level of organization that includes multiple organs working together. For example, the digestive system includes the stomach, intestines, liver, and other organs. This option is too broad and does not refer to a group of similar cells.
• D. an organelle: Organelles are specialized structures within a cell that perform specific functions, such as mitochondria (which produce energy) or ribosomes (which synthesize proteins). Organelles are not groups of cells, so this option is not relevant to the question.

Summary of Key Points:

• A tissue is a group of similar cells working together for a specific function.
• Tissues are part of a hierarchy that includes cells, organs, and systems.
• Organs consist of multiple tissue types, while systems consist of multiple organs.
• Organelles are structures within cells, not groups of cells.

This understanding of biological organization is crucial for grasping more complex concepts in biology, such as how different systems interact and function within an organism.