Step 1: Understanding biofertilizers.
Biofertilizers are living organisms that enrich soil fertility by fixing atmospheric nitrogen. Step 2: Role of Nostoc.
Nostoc is a cyanobacterium capable of fixing atmospheric nitrogen in flooded paddy fields. Step 3: Comparison with other options.
Chlorella is an alga used as food supplement.
Azotobacter and Rhizobium are used mainly in dry fields and leguminous crops respectively. Step 4: Conclusion.
Thus, Nostoc is used as a biofertilizer in paddy fields.