Is a group of cells that work together to perform a specific function.

Tissue.

Explanation:

A tissue is a group of cells which performs a specialized function in the body of a multi-cellular organism.

In a unicellular organism tissue level of organisation does not exist and only a single cell performs all the vital functions like respiration, digestion, reproduction etc. But since a single cell performs all the functions, it is unable to perform all these functions with high efficiency because a single cell can not perform all the functions efficiently.

In contrast to this, a multi-cellular organism has a tissue level of organisation. In such organisms many cells form a group to make a tissue which performs only one specialized function which leads to great efficiency. For example, nervous tissue performs the function of responding to various stimuli only and does not indulge in any other work in the body, cardiac tissue is meant for circulation of blood etc. This is known as division of labor. Each tissue performs its own specialized function for one individual and hence enhance the efficiency of work in the organism.    

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