‘X’ is an essential micronutrient required in leaves and roots. Major function of X is to perform metabolism of urea and uric acid and its deficiency causes leaf tip necrosis. X is-
A. Sulphur
B. Magnesium
C. Zinc
D. Nickel

Hint: Plant nutrients have two categories based on major and minor nutrient uptake. These major and minor nutrients are classified as such because of the quantity of nutrients the plant takes. So, magnesium comes under the secondary or minor plant category. It is because plants require them in smaller quantities than other macronutrients like-nitrogen, phosphorus, potassium.

Complete Explanation:
Option A) Sulphur- Symptoms which plant shows in magnesium deficiency are- interveinal chlorosis of older leaves. It indicates that the leaf veins remain green while the rest of the leaf turns yellow. As magnesium moves from lower leaves to higher or top younger leaves it affects or shows deficiencies in lower leaves first. Magnesium is a limiting factor in plant growth. Hence, Sulphur has nothing to do with it.
So, option A is not correct.
Option B) Magnesium- Magnesium is the energy source behind photosynthesis in plants. Without magnesium chlorophyll cannot capture the sun's energy needed for photosynthesis. Magnesium is located in the enzymes used in the process of chlorophyll synthesis. In other words magnesium provides green pigment to the plant. Magnesium is also required for formation of nucleic acid and also for the metabolism of carbohydrates and it also stabilizes the cell membrane. So, X is none other than magnesium and its deficiency causes leaf tip necrosis.
So, option B is correct.
Option C) Zinc- Zinc is used as a component of chlorophyll formation and some carbohydrates. It also helps in tissue development and formation of auxins which is a plant hormone, helping in stem elongation and growth regulation.
So, option C is not correct.
Option D) Nickel- nickel is also a micronutrient which plays an important role in seed germination, nitrogen fixation and metabolism of urea. It is essential for plant growth.
So, option D is not correct.

Hence, Option B is the correct answer.

Note:
Deficiencies in plants due to magnesium highly affect the plants as magnesium helps in chlorophyll synthesis; the shortage results in poor and stunted plant growth. So, as we discussed some of the factors of magnesium in plants, now let us know why plants require magnesium. A plant requires Mg for chlorophyll synthesis as the Mg is involved in photosynthesis. It serves as an activator for many enzymes required in the plant growth process.