Which of the following best describes a tissue?

A tissue is best described as a collection of similar cells performing a specific function. This definition emphasizes that tissues consist of groups of cells that share a common structure and function, working together to carry out tasks necessary for the body's operation. For example, muscle tissue is made up of muscle cells that contract in unison to facilitate movement. This collective action is what distinguishes tissues from other biological structures.

The other options do not accurately define what a tissue is. The notion of a unit of organs working together pertains more to organ systems, while a group of diverse cell types refers to organs themselves or mixed tissues rather than a uniform tissue type. An individual cell type fails to capture the essence of a tissue as it should involve multiple similar cells cooperating for a common purpose.