Explanation:

- Azolla: This is the correct answer. It's a small, floating aquatic plant that is often used as a biofertilizer in rice paddies. This is due to its ability to fix atmospheric nitrogen through a symbiotic relationship with cyanobacteria.

- Wolfia: Also known as watermeal, it’s one of the smallest flowering plants, but it's not commonly used as a biofertilizer.

- Trapa: Commonly known as water chestnut, it's primarily grown for its edible parts and not used as a biofertilizer.

- Lemna: Known as duckweed, it's used in wastewater treatment and biomass production but not specifically for fertilizing rice fields.