The correct answer is option 1, Calcium. Calcium is not considered a structural component of a plant.

Structural components of a plant refer to the essential elements and compounds that make up the physical structure and support of a plant. These components are necessary for the plant`s overall growth and development.

Option 2, Nitrogen, is a vital structural component of plants. It is a major component of amino acids, proteins, and nucleic acids, all of which are essential for plant growth. Nitrogen plays a crucial role in leaf and stem development and affects the overall size and vigor of the plant.

Option 3, Phosphorus, is another essential structural component of plants. It is an integral part of DNA, RNA, and ATP, which are involved in various cellular processes. Phosphorus is vital for root development, energy transfer, and promoting overall plant growth.

Option 4, Potassium, is also a structural component of plants. It is involved in regulating numerous physiological processes, including water uptake, photosynthesis, enzyme activation, and nutrient transport. Potassium affects the plant`s overall health, root development, and fruit production.

Therefore, the correct answer is option 1, Calcium, as it is not considered a structural component of a plant.