After stamping your replica plates, you return to examine the results and see that there are 100 colonies on the strep nal plate from master plate strain B and no growth on the strep nal plate from master plate strain A. Which strain (A or B) is the streptomycin-resistant master plate strain

An important way of selecting for bacteria carrying specific recombinant DNA is to add a resistance gene (such as antibiotic resistance) to your gene of interest. That means that any bacteria that carry your DNA will be resistant to a specific antibiotic (in this case streptomycin). Therefore, you can grow your bacteria on plates with streptomycin, and in theory, the bacteria will be unable to grow if they don't have the resistance gene and your gene of interest.

Here, strain A does not grow on streptomycin plates, but strain B now has 100 colonies. This suggests strain B can grow on streptomycin, meaning it is resistant.

Strain B is therefore the streptomycin resistant master plate strain.