As per the question , option B i.e. ultrasonic inspection is correct. Let’s have a look in all the process to find the reason.

1) Dye Penetrant test: 

It is widely applied and low-cost inspection method used to check surface-breaking defects in all ninmnin porous(metals, plastics, or ceramics); may be applied to all non-ferrous materials and ferrous materials. Its based on Capillary action where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Now, as capillary action is ineffective as we proceed further from surface it can’t detect small flaws in bulk of material.

So this is not our option.

3) Eddy Current testing: 

This test has similiar principal of Magnetic particle inspection.  This method is useful in detecting near surface defects in conductive materials.

We use an electromagnetic inductor to generate magnetic fields.When this field is introduced in the surface of the test piece, it generates so called “eddy currents” in the material. These currents generate their own magnetic field which resists the initial field created by the inductor. When a discontinuity disturbs the eddy currents, this can be registered by measuring the resulting change in impedance of the coil.

But it’s very difficult to detect the flaws in bulk density to more depth as the changes due to magnetic field can’t be observed properly. So this is not our option.

4) Magnetic Particle Inspection:

This is also a very widely used NDT method mostly for detecting near surface defects in ferromagnetic alloys. The piece can be magnetized by direct or indirect magnetization.

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals.

To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action. But again we can’t observe this in bulk of dense material and it’s not our option.

2) Ultrasonic testing: 

Now , this works on a different principle, it uses sound waves to detect the defects and flaws. Ultrasonic inspection relies on electronic transducers that transmit high-frequency sound waves to a material. These sound waves bounce back images, which reveal key characteristics of a material’s properties. The images created by ultrasonic testing can indicate cracks, weld grooves and fractures, as well as indicate material thickness and moving components. Since it’s obvious that sound can travel and bounce back to more depth of material it is the effective method for detecting flaws in depth of bulk dense material; even small flaws will cause the change in sound frequency and gets detected.

So this is our option.