Micronutrients are nutrients that plants need in very small quantities, but are essential for their growth and development. These include Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn), Molybdenum (Mo), Boron (B), and Chlorine (Cl).
Option 1, Boron (B), is known to be essential for plant growth as it assists in cell wall formation and the transport of sugars.
Option 2, Zinc (Zn), plays a crucial role in DNA synthesis, protein synthesis, and auxin metabolism in plants.
Option 4, Copper (Cu), is involved in photosynthetic electron transport, respiration, and formation of lignin in plants.
However, option 3, Sodium (Na), while it can be beneficial for some plants, it is not considered an essential micronutrient. Sodium can help boost plant metabolism at low concentrations, but it is not vital for plant survival or growth. Therefore, it`s not classified as an `essential` micronutrient for plants, leading to the conclusion that the correct answer is 3 - Sodium.