Answer: organ
All complex living organisms are structurally composed of complex levels of organizations, which are required to perform specialized functions such as respiration, digestion, excretion and others. The biological level of organization initiates with cells which are precursors of life, the group of cells is called as tissue, two or more types of specialized tissues makes up an organ, two or more organs performs a specialized function which makes up an organ system. An organ system makes up an entire organisms, group of organisms make up a population, a population of two or more types of organisms makes up a community, group of two or more community makes up an ecosystem and a group of ecosystem makes up a biosphere.
According to the above description, the level of organization which falls directly after the tissue level is the organ.
