What is the primary definition of a tissue?

The primary definition of a tissue is indeed a group of biological cells that perform a specialized function. Tissues are the building blocks of organs and are composed of similar cells that work together to carry out specific activities essential for the overall function of an organism. For example, muscle tissues are responsible for movement, nervous tissues transmit signals, and epithelial tissues cover body surfaces and organs.

The other choices offer related concepts but do not capture the essential definition of tissue itself. Defining a tissue as a type of organ overlooks the fact that tissues are actually the precursors to organs, which are composed of various types of tissues functioning together. The definition of tissue as a layer of cells that protects body surfaces is specific to epithelial tissue but does not encompass all tissue types. Lastly, describing a tissue as a collection of organs is incorrect, as organs are formed from multiple tissues working in concert rather than a tissue being the sum of several organs. Thus, the correct definition captures the fundamental aspect of tissues as a cohesive group of cells with a shared function.