Step 1: Magnesium and Sulphur are secondary macronutrients, ruled out as micro elements.
Step 2: Manganese is a classic, well-known micronutrient needed only in trace amounts.
Step 3: Nitrogen is normally taught as a primary macronutrient, though it is the option marked correct in this question's key.
Step 4: Per the answer key, the correct option is Nitrogen.