Groups of organs that work together to complete a process in the body are called:

cells
tissues
nerves
systems

The answer here would be tissues.

for example,The Epithelial Tissue  , or skin, is involved with protection, secretion, absorption, and filtration, it protects the body from dirt, dust, and other microbes that may be harmful to the body. 

The Connective Tissue is the most abundant and widely distributed tissue of them all, their job is to provide support and protection for the body. This includes ordinary loose connective tissue, fat tissue, dense fibrous tissue, cartilage, bone, blood, and lymph, hence the reason why its considered abundant.

Lastly there is the Muscle tissue: Skeletal, Smooth, and Cardiac:  There are three types of muscle tissue: skeletal, smooth, and cardiac.  Skeletal muscle is a voluntary type of muscle tissue that is used in the contraction of skeletal parts.  Smooth muscle is found in the walls of internal organs and blood vessels.  It is an involuntary type.  The cardiac muscle is found only in the walls of the heart and is involuntary in nature.