**Core Concept**
The most commonly used method for testing antibiotic susceptibility of bacteria involves evaluating the minimum concentration of an antibiotic required to inhibit the growth of a particular bacterial species. This process is crucial in guiding antibiotic therapy and preventing the emergence of antibiotic-resistant pathogens.

**Why the Correct Answer is Right**
The correct answer involves the Kirby-Bauer disk diffusion method, which relies on the principle of antibiotic diffusion through an agar plate. Bacteria are grown on the plate, and antibiotic disks are placed on its surface. The zone of inhibition around each disk is measured, and the diameter of the zone is correlated with the susceptibility of the bacteria to the antibiotic. This method is widely used due to its simplicity, speed, and ease of interpretation.

**Why Each Wrong Option is Incorrect**
**Option A:** The agar dilution method is another technique used for antibiotic susceptibility testing, but it is more labor-intensive and less commonly used than the Kirby-Bauer disk diffusion method.
**Option B:** The E-test is a type of agar diffusion method, but it is not as widely used as the Kirby-Bauer method.
**Option C:** The broth microdilution method is a more accurate and sensitive technique, but it is more time-consuming and less commonly used in clinical settings.

**Clinical Pearl / High-Yield Fact**
It's essential to note that antibiotic susceptibility testing should be performed using standardized methods, such as the Kirby-Bauer disk diffusion method, to ensure accurate and reliable results.

**Correct Answer:** C.