The aquatic plant commonly used as biofertilizer in waterlogged rice fields is:

1. A

   Lemna

   Lemna claims to be a free-floating aquatic plant often found in stagnant water bodies. However, it lacks the symbiotic relationship with nitrogen-fixing cyanobacteria that makes a true biofertilizer for rice.

2. B

   Azolla

   Azolla claims to be a floating fern that hosts Anabaena azollae, a cyanobacterium capable of fixing atmospheric nitrogen. This unique biological partnership enriches waterlogged rice fields with essential nutrients, making it the recognized biofertilizer and the correct official key.

3. C

   Wolffia

   Wolffia claims to be the smallest flowering aquatic plant on Earth, sometimes consumed as food in certain regions. It is not traditionally utilized or recommended as a primary biofertilizer for wetland paddy cultivation.

4. D

   Trapa

   Trapa claims to be an aquatic floating-leaved plant cultivated for its edible nuts, commonly known as water chestnut. While it grows in aquatic environments, it does not serve the agronomic function of nitrogen fixation in rice paddies.

Summary. Official key is (b) Azolla. This aquatic fern forms a vital symbiotic association with nitrogen-fixing blue-green algae. Farmers deliberately introduce it into waterlogged paddy fields to enhance soil fertility naturally. Other options like Lemna, Wolffia, and Trapa lack this specialized nitrogen-fixing capability. Thus, only option (b) correctly identifies the standard biofertilizer used in rice cultivation.