Antibiotic effectiveness via zone of inhibition Steps:

• Observe the agar plate: shaded areas indicate bacterial growth, clear zones show inhibition by antibiotics.
• Measure clear zones around each disk: larger zones mean the antibiotic diffuses farther and kills more bacteria.
• Compare zones for antibiotics A, B, C, D: identify the largest clear area.
• Select the antibiotic with the widest inhibition zone as most effective.

Why A is correct:

• A shows the largest clear zone, indicating strongest bacterial inhibition per disk diffusion test principles.

Why the others are wrong:

• B has a smaller zone, allowing more bacterial growth nearby.
• C has minimal or no clear zone, showing little to no effectiveness.
• D permits bacterial growth up to or overlapping the disk, indicating resistance.

Final answer: A