Correct Option: B. Magnesium Detailed Explanation: When a young plant exhibits yellow leaves, this condition is known as chlorosis. Chlorosis can occur due to various nutrient deficiencies, and understanding which nutrients are essential for plant health is crucial for diagnosing the problem.

1. Understanding Nutrient Roles:
2. Calcium (Option A): Calcium is vital for cell wall structure and stability. It helps in the formation of new cells and is essential for root development. A deficiency in calcium can lead to symptoms like blossom end rot in tomatoes and tip burn in lettuce, but it does not typically cause yellowing of leaves.
3. Magnesium (Option B): Magnesium is a central component of chlorophyll, the molecule responsible for photosynthesis. A deficiency in magnesium often leads to interveinal chlorosis, where the areas between the veins of the leaves turn yellow while the veins themselves remain green. However, in young plants, magnesium deficiency is less common compared to other nutrients.
4. Potassium (Option C): Potassium is crucial for various plant processes, including water regulation, enzyme activation, and photosynthesis. A deficiency in potassium can lead to yellowing of the leaf margins and overall poor plant health.
5. Boron (Option D): Boron is important for cell division and the development of new tissues. A deficiency in boron can lead to yellowing and death of young leaves, as well as poor root development.
6. Why Magnesium is NOT the Likely Deficiency:
7. In young plants, magnesium is often present in sufficient quantities due to its mobility in the soil and its role in chlorophyll production. While magnesium deficiency can cause yellowing, it is less likely to be the primary cause in young plants compared to deficiencies in calcium, potassium, or boron.
8. Additionally, magnesium is often supplied in adequate amounts through fertilizers and soil amendments, making it less likely to be the nutrient that is deficient in this scenario.

Why the Other Options are Incorrect or Weaker:

• Option A (Calcium): Calcium deficiency can lead to yellowing, particularly in young tissues. Since calcium is crucial for cell wall integrity and growth, a deficiency would likely manifest in young plants, making this option a strong contender for causing yellow leaves.
• Option C (Potassium): Potassium deficiency is well-known for causing yellowing of leaves, especially at the edges. This is a common issue in many plants, particularly in young ones that are still developing their root systems and may not be able to uptake sufficient potassium from the soil.
• Option D (Boron): Boron deficiency can lead to yellowing and death of young leaves, as it is essential for new growth and development. Young plants are particularly susceptible to boron deficiency, making this option a strong candidate for causing yellow leaves.

Summary of Key Points:

• Chlorosis: Yellowing of leaves often indicates nutrient deficiencies.
• Magnesium: While important for chlorophyll, it is less likely to be deficient in young plants compared to calcium, potassium, or boron.
• Calcium, Potassium, and Boron: All can cause yellowing in young plants, making them more likely culprits for the observed symptoms.
• Diagnosis: Understanding the role of each nutrient helps in diagnosing plant health issues effectively.

Revision Summary:

• Yellow leaves in plants can indicate nutrient deficiencies, commonly referred to as chlorosis.
• Magnesium is less likely to be deficient in young plants compared to calcium, potassium, or boron.
• Calcium is essential for cell structure, potassium for water regulation, and boron for new growth.
• Always consider the specific roles of nutrients when diagnosing plant health issues.