Nitrogen fixation occurs in:

**The reasoning** Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) that plants can actually use. This happens thanks to special bacteria called **Rhizobium** that live in the **root nodules of leguminous plants** (like beans, groundnut, soybeans). Here's the deal: These bacteria form a symbiotic relationship with legume roots. The plant provides them sugar for energy, and in return, the bacteria convert nitrogen from air trapped in soil into usable compounds. That's why farmers plant legumes to naturally enrich soil with nitrogen! **Why the wrong options tempt you** - **B (Stems)**: Stems transport nutrients but don't fix nitrogen. The bacteria need the protected environment of root nodules. - **C (Leaves)**: Leaves do photosynthesis, not nitrogen fixation. Don't confuse the two processes! - **D (Flowers)**: Flowers are for reproduction. They have zero role in nutrient conversion. **Quick takeaway** Remember: **"Rhizobium bacteria in root nodules = nitrogen fixation in legumes."** Think groundnut roots with little bumps (nodules) — that's your nitrogen factory! 🌱