The Ti plasmid is found in

The correct option is Agrobacterium.

Step-by-step Explanation:

1. Understanding the Ti Plasmid:
The Ti (Tumor-inducing) plasmid is a double-stranded circular plasmid found in certain bacteria. It is widely known for its ability to transfer a specific segment of its DNA, known as the T-DNA (transfer DNA), into the host plant's genome, leading to the formation of crown gall disease (tumors).

2. Identifying the Host Bacterium:
The Ti plasmid is naturally harbored by the soil bacterium Agrobacterium tumefaciens (now often classified under Rhizobium radiobacter). Therefore, the correct source among the given options is Agrobacterium.

3. Analyzing the Other Options:
- Yeast as a 2mm plasmid: Yeast contains a different, well-characterized plasmid known as the 2-micron (2µm) circle, which is not a Ti plasmid.
- Rhizobium of the roots of leguminous plants: While related to Agrobacterium, symbiotic Rhizobium species typically contain Sym (symbiotic) plasmids rather than Ti plasmids.
- Azotobacter: This is a free-living, nitrogen-fixing bacterium and does not naturally contain the Ti plasmid.