An organ, in any multicellular organism, would BEST be described as

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several types of cells working together for a common purpose.
several types of cells working together for a common purpose.

a group of the same kind of cells with a common structure and function.
a group of the same kind of cells with a common structure and function.

a structure containing a single cell that is part of a large system.
a structure containing a single cell that is part of a large system.

a structure made of two or more tissues functioning together for a common purpose.

a structure made of two or more tissues functioning together for a common purpose.