Final answer:
The best region of the protein to use for designing a reverse-translated probe in a colony hybridization experiment is a highly conserved region across different species.
Step-by-step explanation:
In a colony hybridization experiment, a reverse-translated probe is used to identify DNA containing the sequence of interest in the protein. The best region of the protein to use for designing the reverse-translated probe would be the region that is highly conserved among different species. This is because conserved regions indicate important functional regions of the protein that are less likely to vary across species. By targeting a conserved region, the probe is more likely to successfully hybridize with the target DNA, allowing for accurate identification of colonies containing the specific cDNA of interest.