Which of the following is NOT one of the essential plant nutrients?

Silicon is classified as a beneficial element in plant nutrition, but it is not considered one of the essential nutrients. Essential plant nutrients are those that are necessary for a plant's growth and reproduction, and without which the plant cannot complete its life cycle. These typically include macronutrients such as nitrogen, phosphorus, potassium, as well as micronutrients like manganese.

Carbon is an essential nutrient because it is a key component of carbohydrates, proteins, and fats, which are vital for plant structure and energy. Nitrogen is also essential, frequently referred to as a primary nutrient because it is crucial for amino acids, proteins, and nucleic acids. Manganese serves as a micronutrient that plays important roles in photosynthesis and various enzymatic processes. Since silicon does not meet the criteria of being required for all plants for growth and reproduction, it does not fall into the category of essential nutrients, making it the correct answer in this context.