Explanation:

Micronutrients are essential elements that are required by plants in small amounts for their growth and development. They play a vital role in various physiological and biochemical processes in plants. Out of the given nutrients, Copper (Cu) and Molybdenum (Mo) are considered micronutrients.

1. Copper (Cu):
- Copper is an essential micronutrient required by plants for various enzymatic reactions.
- It plays a crucial role in the synthesis of chlorophyll and lignin, which are important for photosynthesis and cell wall formation, respectively.
- Copper also acts as a cofactor for several enzymes involved in various metabolic processes.

2. Magnesium (Mg):
- Magnesium is not considered a micronutrient, but rather a macronutrient required in relatively larger quantities by plants.
- It is an essential component of chlorophyll, the pigment responsible for photosynthesis.
- Magnesium also plays a role in the activation of enzymes and the synthesis of nucleic acids.

3. Molybdenum (Mo):
- Molybdenum is an essential micronutrient required by plants in small amounts.
- It is involved in the conversion of nitrate to ammonia during nitrogen metabolism.
- Molybdenum also plays a role in the synthesis of enzymes involved in nitrogen fixation and nitrate reduction.

4. Sulphur (S):
- Sulphur is not considered a micronutrient, but rather a macronutrient required in relatively larger quantities by plants.
- It is a component of certain amino acids, vitamins, and coenzymes.
- Sulphur is also involved in the synthesis of proteins and chlorophyll.

Conclusion:
From the given nutrients, Copper (Cu) and Molybdenum (Mo) are considered micronutrients. Therefore, option 'C' is the correct answer. Magnesium (Mg) and Sulphur (S) are macronutrients required in larger quantities by plants.