Explanation:

Definition of Bio-fertilizer:
Bio-fertilizers are a type of fertilizers that contain living microorganisms, such as bacteria, fungi, and algae, which enhance the nutrient availability and uptake by plants. These microorganisms establish a symbiotic relationship with the plant roots and promote their growth by fixing atmospheric nitrogen, solubilizing phosphorus, producing growth-promoting substances, and suppressing plant pathogens.

Options:
a) Bacillus thuringiensis
b) Anabaena
c) Nostoc
d) Rhizobium

Explanation of each option:

a) Bacillus thuringiensis:
Bacillus thuringiensis (Bt) is a bacterium that produces crystal proteins toxic to certain insect pests. It is widely used as a biological insecticide to control pests in agriculture and forestry. However, it is not used as a bio-fertilizer as it does not have the ability to fix nitrogen or solubilize phosphorus.

b) Anabaena:
Anabaena is a genus of filamentous cyanobacteria that are capable of fixing atmospheric nitrogen. They form symbiotic associations with certain plants, such as Azolla (a floating aquatic fern), and provide them with nitrogen. Anabaena is commonly used as a bio-fertilizer in paddy fields to enhance nitrogen availability for rice plants.

c) Nostoc:
Nostoc is another genus of filamentous cyanobacteria that can fix atmospheric nitrogen. Like Anabaena, Nostoc forms symbiotic associations with certain plants and contributes to nitrogen fixation. It is also used as a bio-fertilizer in paddy fields.

d) Rhizobium:
Rhizobium is a genus of soil bacteria that form symbiotic associations with leguminous plants (plants belonging to the family Fabaceae). These bacteria infect the root nodules of legumes and convert atmospheric nitrogen into a form that can be utilized by the plant. Rhizobium is widely used as a bio-fertilizer for leguminous crops like peas, beans, and clover.

Conclusion:
Among the given options, Bacillus thuringiensis is not used as a bio-fertilizer because it does not possess the ability to fix nitrogen or solubilize phosphorus. The other options, Anabaena, Nostoc, and Rhizobium, are commonly used as bio-fertilizers due to their nitrogen-fixing capabilities.