The correct answer is option 'A' - Sulphur.

Sulphur is a macronutrient that is essential for the growth and development of plants. It is considered a macronutrient because plants require it in relatively large quantities compared to micronutrients. Here is a detailed explanation of why sulphur is classified as a macronutrient:

1. Importance of Sulphur:
Sulphur plays an important role in various physiological and metabolic processes in plants. It is a constituent of certain amino acids, proteins, and enzymes. It is also involved in the formation of chlorophyll, which is necessary for photosynthesis. Sulphur is essential for the synthesis of vitamins and coenzymes, and it also contributes to the structure and stability of cell membranes.

2. Macronutrient Characteristics:
Macronutrients are nutrients that plants require in large amounts for their growth and development. They are classified as macronutrients based on the quantity required, not because they are more important than micronutrients. Sulphur is one of the macronutrients because it is needed by plants in relatively large quantities, typically in the range of 0.2-5% of the plant's dry weight.

3. Sulphur Deficiency and Symptoms:
When plants do not receive an adequate supply of sulphur, they may exhibit symptoms of deficiency. These symptoms include yellowing of leaves (chlorosis), stunted growth, reduced protein synthesis, and decreased yield. Sulphur deficiency can limit plant growth and productivity, making it essential to ensure an adequate supply of this macronutrient.

4. Sources of Sulphur:
Plants obtain sulphur primarily from the soil. It is present in both organic and inorganic forms. Organic sources include decomposing organic matter, manure, and compost. Inorganic sources include sulphate salts, such as potassium sulphate or magnesium sulphate, which can be added as fertilizers to supplement the sulphur content in the soil.

In conclusion, sulphur is classified as a macronutrient because it is required by plants in relatively large amounts for their growth and development. It plays a vital role in various physiological and metabolic processes, and its deficiency can have detrimental effects on plant growth and productivity.