An organ is a

Options

A. group of two or more kinds of tissues united structurally and coordinated together to perform an activity
B. group of similar cells that function together to perform an activity
C. multilayered sheet of cells
D. solid structure formed of embryonic mesoderm

Answer

**Step-by-Step Solution:**

1. **Definition of an Organ**: An organ is defined as a group of two or more different types of tissues that are united and coordinated to perform a specific activity or function.

2. **Understanding Tissues**: Tissues are formed when similar cells group together. These tissues can be of various types, such as muscular tissue, nervous tissue, epithelial tissue, etc.

3. **Hierarchy of Biological Organization**: The biological organization starts from the smallest unit, which is the cell. Cells come together to form tissues, and when different types of tissues work together, they form an organ.

4. **Examples of Organs**: Common examples of organs include the heart, lungs, kidneys, and liver. Each of these organs is made up of multiple types of tissues working together to perform their specific functions.

5. **Organ Systems**: When multiple organs work together to perform a broader function, they form an organ system, such as the circulatory system or the excretory system.

6. **Incorrect Options**:
- Option 2 describes a group of similar cells, which refers to tissues, not organs.
- Option 3 describes a multi-layered sheet of cells, which does not define an organ.
- Option 4 mentions solid structures formed from embryonic mesoderm, which is not a comprehensive definition of an organ.

7. **Conclusion**: The correct definition of an organ is that it is a group of two or more kinds of tissues that are united and coordinated to perform a specific activity. Therefore, the correct answer is option 1.

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