The introduction of t-DNA into plants involves:
Allowing the plant roots to stand in water
Infection of the plant by Agrobacterium tumefaciens
Altering the pH of the soil then heat-shocking the plants
Exposing the plants to cold for a brief period

Agrobacterium tumefaciens a plant pathogenic bacterium that carries Ti plasmid. It can transfer a particular segment of the tumor-inducing (Ti) plasmid into the nucleus of infected cells. The transferred T-DNA is then integrated into the host genome and transcribed with it. This ability of Agrobacterium tumefaciens to transfer the T-DNA in the host genome is explored in genetic engineering to transfer the desired DNA segment, carrying the gene of the interest, into the genome of selected organisms. Thus, the correct answer is option C (Infection of the plant by Agrobacterium tumefaciens).