Option 1: Carbon is not considered a micronutrient for plants because it is a macronutrient that plants require in large quantities for photosynthesis.
Option 2: Oxygen is also not considered a micronutrient for plants because it is obtained from the air and water, and is required for respiration and energy production.
Option 3: Nitrogen is not classified as a micronutrient either, as it is needed in relatively large amounts by plants for protein synthesis and growth.
Option 4: Boron is indeed a micronutrient for plants. It plays a crucial role in several plant functions, including cell wall formation, hormone regulation, and carbohydrate metabolism. Although plants require boron in small amounts, it is still essential for their overall growth and development.
Therefore, the correct answer is option 4.