Bio-fertilizers are

Correct option is D

The correct answer is (D) Culture of micro-organisms

Explanation:
• Bio-fertilizers are preparations containing living cells or latent cells of efficient strains of microorganisms (bacteria, fungi, or algae).
• When applied to seeds, plant surfaces, or soil, they colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients.
• Unlike chemical fertilizers, they do not contain nutrients directly but help in Nitrogen fixation (e.g., Rhizobium, Azotobacter), Phosphate solubilization (e.g., Pseudomonas), or mobilizing nutrients (e.g., Mycorrhizae).
• They are eco-friendly, cost-effective, and improve soil health over time.

Information Booster:
• Rhizobium: Specific for legumes; fixes nitrogen in root nodules.
• Azotobacter: Free-living nitrogen fixer; used for crops like wheat and maize.
• Azolla: A water fern that lives in symbiosis with Anabaena (blue-green algae), widely used in rice cultivation.
• VAM (Vesicular Arbuscular Mycorrhiza): A fungus that helps in phosphorus absorption and water uptake.

Additional Knowledge:
• Organic manure (Option A): A broad term including any decayed organic matter used as fertilizer.
• Green manure (Option B): Specifically refers to crops (like Sunnhemp or Dhaincha) grown and plowed back into the soil while still green.
• FYM (Farm Yard Manure) (Option C): A decomposed mixture of dung, urine, and litter/bedding from livestock.