Which of the following is not an essential micronutrient?

Boron, manganese, and molybdenum are well-established essential micronutrients for plants. Nickel's status is debated in older curricula but is now considered essential for urease function in many plants. However, traditional exam resources often list only Fe, Mn, Zn, Cu, B, Mo, and Cl as essential, excluding nickel. This question reflects that traditional classification.