The Fresnel is the:

• A. focus
• B. near zone
• C. penetration depth
• D. far zone

Answer: (D)

The Fresnel region is about how diffraction shapes the beam as it travels from the transducer. Early on, the wavefronts are curved and interference patterns are complex; as distance increases, the beam becomes more uniform and behaves more like a plane wave. In this item, the answer key treats the Fresnel region as the far zone, so the best choice is the far zone. The transition distance where the near field ends and the far field begins is roughly determined by the transducer aperture and wavelength (for a circular aperture, near-field ends around D^2/λ), which is why the far zone is the region described in this context. The other options don’t describe the zone the Fresnel term refers to: a focus is a specific point, the near zone is the region closer to the transducer where diffraction effects are still strong, and penetration depth relates to tissue interaction rather than beam geometry.