ANS:B - Phosphorous

The nutrient in fertilizer that makes plant stems stronger and increases branching is Potassium. Potassium (K) plays a crucial role in various physiological processes in plants, including:

1. Cellular Strength: Potassium helps regulate the opening and closing of stomata, which are small pores on the leaves responsible for gas exchange. This regulation helps maintain turgor pressure within the plant cells, enhancing their structural integrity and making stems stronger.
2. Enzyme Activation: Potassium is involved in activating many enzymes responsible for various metabolic processes within the plant. These enzymes play key roles in cell division, protein synthesis, and carbohydrate metabolism, which are essential for promoting healthy growth and development, including branching.
3. Water Regulation: Potassium helps regulate water uptake and transport within the plant. Proper water regulation is essential for maintaining turgidity in plant cells, which contributes to stem strength and prevents wilting. Additionally, potassium deficiency can lead to water stress, affecting overall plant growth and branching.
4. Disease Resistance: Adequate potassium levels in plants improve their resistance to diseases and environmental stresses. Stronger, healthier plants with sufficient potassium are better equipped to withstand biotic and abiotic stresses, which can promote branching and overall plant vigor.

While nitrogen (N), phosphorus (P), and calcium (Ca) are also essential nutrients for plant growth, they primarily contribute to other aspects of plant development, such as leaf growth, root development, and fruit production, rather than directly influencing stem strength and branching. Therefore, the correct answer is Potassium.