Biofertilizers are living organisms that enrich the soil's nutrient content and promote plant growth. They are eco-friendly, cost-effective, and sustainable alternatives to chemical fertilizers. The given options are all examples of biofertilizers, except for Agrobacterium.

Agrobacterium:

Agrobacterium is a bacterium that causes plant diseases, such as crown gall disease in plants. It is not a biofertilizer, but rather a plant pathogen that infects the host plant's tissue and causes the formation of tumors.

Rhizobium:

Rhizobium is a soil bacterium that forms a symbiotic relationship with leguminous plants, such as soybean, peas, and clover. It fixes atmospheric nitrogen into a form that can be used by the plant, thereby improving the soil's fertility.

Nostoc:

Nostoc is a blue-green alga that fixes atmospheric nitrogen and enriches the soil's nutrient content. It forms symbiotic associations with certain plants, such as Azolla, and is used as a biofertilizer in rice fields.

Mycorrhiza:

Mycorrhiza is a symbiotic association between fungi and the roots of most plants. It enhances the plant's nutrient uptake, especially phosphorus, by increasing the root surface area and soil exploration.

In conclusion, Agrobacterium is not a biofertilizer, as it causes plant diseases, unlike the other options given.

Agrobacterium is not a biofertilizer. It causes crown gall disease in plants because of which it is most extensively used in biotechnology for the gene transfer and its expression...So, the correct answer is option A.