Which of the following best describes an organelle?

An organelle is best described as a functional part of a cell. Organelles are specialized structures within a cell that perform distinct functions necessary for the cell's life processes. For instance, the mitochondria are involved in energy production, while ribosomes are responsible for protein synthesis. This specialization allows cells to efficiently carry out the various tasks required for growth, maintenance, and reproduction.

The other options, while they refer to biological entities, do not accurately capture the definition of an organelle. A group of cells refers to tissue, which is a higher level of organization; a type of tissue describes a collection of similar cells working together, which is a concept distinct from the individual organelles that make up a cell; and a molecule is a smaller unit composed of atoms, which does not describe the complex functions attributed to organelles within the cellular framework. Thus, the choice highlighting the functional role within the cell is the most accurate description of an organelle.