If you notice in the graph for antibiotic A, the number of bacteria actually INCREASES as time increases after the antibiotic was given. In the second graph, the amount of bacteria increases just a little bit (likely as the antibiotic sets in) and then decreases until no bacteria is left at all. This means that the antibiotic was the most successful because not only did the amount of bacteria decrease over time, but also all of the bacteria were eventually killed.
The last graph is shown as no antibiotic given. This is a graph showing the control group. There is always a control group in an experiment where nothing is done to the group. This is used to compare the results in the end of the experiment.
