ANS:B - Detection of defects in metal.

The applications of ultrasonic testing include:

1. Detection of defects in metal: Ultrasonic testing is commonly used to detect various types of defects in materials such as cracks, voids, inclusions, and discontinuities. By analyzing the reflections of ultrasonic waves from these defects, it's possible to identify their presence and assess their size, shape, and location within the material.
2. Measurement of material thickness: Ultrasonic testing can also be used to measure the thickness of materials, particularly in cases where access to both sides of the material is limited. By sending ultrasonic waves through the material and measuring the time taken for the waves to travel through it, the thickness of the material can be accurately determined.