Final answer:
The experiment with leu- and leu+ bacteria suggests that conjugation is the method of gene transfer, as it requires direct cell-to-cell contact, which only occurs when the bacteria are mixed together in a flask and not separated by a U-tube.
Step-by-step explanation:
When leu- bacteria are mixed with leu+ bacteria in a flask resulting in all bacteria becoming leu+, but this does not occur when the leu- and leu+ bacteria are separated by a U-tube, it suggests that the interaction requires direct contact between the two types of bacteria. Therefore, the process is most likely conjugation, where DNA is transferred directly from one prokaryote to another by means of a conjugation pilus, requiring close contact between the bacteria.
In contrast, transformation involves the uptake of free DNA from the environment, which wouldn't require direct cell-to-cell contact and could potentially occur across a U-tube barrier if small DNA pieces were to diffuse through. Transduction involves the transfer of DNA between bacteria via bacteriophages and also would not necessarily need direct contact between bacteria. Since the leu- cells do not become prototrophic when physically separated from the leu+ cells, the most likely mechanism for DNA transfer is conjugation.