Correct option is D
Ultrasonic pulse velocity testing is one of the most frequently used non-destructive testing (NDT) methods. This technique uses high-frequency sound waves to evaluate the properties of materials and detect defects or inconsistencies within them.
Rebound hammer: Used to assess the hardness or surface strength of concrete, not as commonly used as ultrasonic testing.
Acoustic mapping: Involves mapping the sound propagation through materials, less commonly used compared to ultrasonic testing.
Radar: Ground-penetrating radar is used for subsurface investigations but is less frequent for general material testing compared to ultrasonic methods.
Therefore, the correct answer is (d) Ultrasonic pulse velocity.
