4. What non-destructive testing method uses sound waves to detect cracks and flaws?
   a) Magnetic particle inspection
   b) Radiographic inspection
   c) Fluorescent inspection
   d) Ultrasonic inspection.

Ultrasonic inspection, the answer you’re looking for, utilizes high-frequency sound waves to detect defects within materials. This method allows technicians to identify cracks, voids, and inclusions without causing any harm to the tested object, making it perfect for critical structures like aircraft components and pipelines. The sound waves travel through the material, and by analyzing the echoes, technicians can assess the integrity of the component.

In real-world applications, ultrasonic testing is widely used in industries such as construction, aerospace, and manufacturing. Its effectiveness in maintaining safety standards is invaluable, especially in sectors where failures can lead to catastrophic consequences. By employing this method, engineers can ensure that components meet rigorous safety and quality requirements, ultimately saving lives and reducing costs associated with repairs.