Introduction: In this question, we are asked to identify the biofertilizer that has been reported to increase rice yields by over 50%. The correct answer is option D, Azolla pinnata. Let's explore why Azolla pinnata is an effective biofertilizer for rice cultivation.

Azolla pinnata:
- Azolla pinnata is a small aquatic fern that has a symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena azollae.
- This symbiotic association allows Azolla to fix atmospheric nitrogen, making it a valuable biofertilizer for rice cultivation.

Benefits of Azolla pinnata as a biofertilizer for rice:
1. Nitrogen fixation:
- Azolla pinnata can fix atmospheric nitrogen due to its symbiotic association with Anabaena azollae.
- Nitrogen is an essential nutrient for plants, and rice requires a significant amount of nitrogen for optimal growth and yield.
- By fixing nitrogen, Azolla pinnata provides a readily available source of nitrogen to the rice plants, leading to increased yields.

2. Organic matter and nutrients:
- Azolla pinnata grows rapidly and forms a dense mat on the water surface, acting as a green manure.
- When Azolla is incorporated into the soil, it decomposes, releasing organic matter and nutrients.
- This organic matter improves soil fertility, enhances nutrient availability, and promotes the growth of beneficial soil microorganisms.

3. Weed suppression:
- The dense mat formed by Azolla pinnata on the water surface acts as a natural weed suppressor.
- It shades out competing weed species, reducing their growth and competition with rice plants.
- This weed suppression effect helps in maintaining the health and productivity of the rice crop.

4. Water and temperature regulation:
- The floating mat of Azolla pinnata reduces water evaporation from the rice fields, conserving water.
- It also acts as an insulating layer, reducing temperature fluctuations in the soil and providing a favorable environment for the rice plants.

Conclusion:
Azolla pinnata is an effective biofertilizer for rice cultivation due to its ability to fix atmospheric nitrogen, provide organic matter and nutrients, suppress weeds, and regulate water and temperature. Farmers have reported over 50% higher yields of rice when using Azolla pinnata as a biofertilizer.