The correct answer is option 'D', Potassium.

Potassium is an essential nutrient for plant growth and development, but it is not a structural component of plants. It plays a crucial role in various physiological processes, but it does not contribute directly to the physical structure of plant tissues.

Here is an explanation of each nutrient and its role in plant growth:

a) Nitrogen:
- Nitrogen is a vital nutrient for plants and is required in large quantities.
- It is a component of amino acids, proteins, nucleic acids, and chlorophyll.
- Nitrogen is involved in various metabolic processes and is essential for plant growth and development.
- It contributes to the structure of plant tissues, especially proteins and chlorophyll.

b) Calcium:
- Calcium is an essential macronutrient for plants.
- It is a structural component of cell walls and contributes to their strength and rigidity.
- Calcium also plays a role in maintaining the integrity of cell membranes and in cell signaling.
- It is involved in various physiological processes, including nutrient uptake, enzyme activity, and hormone regulation.

c) Phosphorus:
- Phosphorus is a macronutrient required for plant growth and development.
- It is a structural component of nucleic acids, ATP (adenosine triphosphate), and phospholipids.
- Phosphorus is involved in energy transfer, DNA replication, and cell division.
- It contributes to the structure and function of plant cells and is essential for root development, flowering, and fruiting.

d) Potassium:
- Potassium is a macronutrient necessary for plant growth and development.
- It plays a vital role in various processes such as osmoregulation, enzyme activation, and protein synthesis.
- Potassium is involved in maintaining water balance, regulating stomatal opening and closing, and facilitating nutrient uptake.
- While potassium is essential for plant health, it does not directly contribute to the structural components of plants such as cell walls or tissues.

In summary, while all the mentioned nutrients (nitrogen, calcium, phosphorus, and potassium) are essential for plant growth, only potassium is not a structural component of plants. It plays a significant role in various physiological processes but does not contribute directly to the physical structure of plant tissues.