What we see around three of the antibiotic disks is the area in which the antibiotics stopped the bacteria from growing. It's called a zone of inhibition and it creates an area where the bacteria haven't grown enough to be visible. So, in order to understand which is the most effective antibiotic we must look at its zones of inhibition and find the one with the biggest zone. Therefore, the correct answer is 3, as it is the one with the bigger zone of inhibition.