In biology, the structural organisation of a living body progresses through different levels. A simple sequence is given as Cell, Tissue, Organ, ?. Choose the most appropriate term from the options that logically completes this sequence of levels of organisation.

Introduction / Context:
This question examines your understanding of the levels of structural organisation in biology. From the smallest functional unit to more complex structures, living organisms are built in a hierarchical order. Knowing this order is essential for topics in school biology, human anatomy, and physiology.

Given Data / Assumptions:

• The given sequence is: Cell, Tissue, Organ, ?
• Each term refers to a level of organisation in a multicellular organism.
• We must select the most logical next level that comes immediately after an individual organ.

Concept / Approach:
In biology, the typical progression of structural organisation in multicellular organisms is:

• Cell: smallest structural and functional unit of life.
• Tissue: group of similar cells performing a common function.
• Organ: structure made of different tissues working together.
• Organ system: group of organs working together for a major function.
• Organism: complete living individual.

Therefore, the step that follows a single organ is the organ system level, not directly the whole organism.

Step-by-Step Solution:
Step 1: Identify the first three terms: Cell, Tissue, Organ. These are successive levels in increasing order of complexity. Step 2: Recall the complete hierarchy: Cell → Tissue → Organ → Organ system → Organism. Step 3: Since the sequence stops at Organ, we must choose the structure that is formed by a group of organs working together. This is known as an Organ system. Step 4: Examine the options. Respiratory system, Digestive system, and Nervous system are all examples of particular organ systems, while Organ system is the general term for that level of organisation. Step 5: The question sequence is general, not specific to any one function. Therefore, the most appropriate and general level that completes the sequence is Organ system.

Verification / Alternative check:
You can quickly verify by expanding any example: Cells of the heart make cardiac tissue, tissues form the heart organ, the heart along with blood vessels forms the circulatory organ system, and many organ systems together form the organism. This confirms that the level following Organ must be Organ system.

Why Other Options Are Wrong:
Respiratory system is a specific example of an organ system, but the sequence is stated in general terms. Using a specific system breaks the general pattern. Digestive system is also a specific organ system rather than the general name of the level of organisation. Nervous system is another particular organ system, again too specific for the given general sequence. Cell division is a biological process that describes how cells multiply, not a structural level in the hierarchy.

Common Pitfalls:
Students sometimes jump from Organ directly to Organism and forget that organs are grouped into organ systems first. Another mistake is to be attracted by familiar examples like Digestive system instead of selecting the more appropriate general term Organ system. Remembering the full hierarchy in order helps avoid these errors.

Final Answer:
The level that follows Organ in the hierarchy of biological organisation is the Organ system.