The correct option is C Only iii is incorrect
Essential elements in plants are divided into 4 broad categories:
i. Some essential elements are structural components of cells. Example- carbon, hydrogen, oxygen & nitrogen.
ii. Some essential elements are components of energy related compounds. Example- Magnesium, phosphorus.
iii. Some essential elements can activate or inhibit enzymes. Example- Zinc, molybdenum.
iv. Some essential elements can change osmotic potential of a cell. Example- Potassium.
Since all the essential elements are not components of energy related compounds statment iii is incorrect.