Final answer:
Reflection occurs at a boundary when there is a difference in acoustic impedance between two different media; the greater this difference, the more pronounced the reflection. Option a.
Step-by-step explanation:
For reflection to take place at the boundary between two different media, the situation that must exist is a. The border of two different tissues must have different impedances. Reflection at boundaries is a consequence of differences in a characteristic called the acoustic impedance Z of each substance.
Acoustic impedance, which is essential in fields such as medical ultrasonography, is defined as Z = pv, where p is the density of the medium and v is the speed of sound through the medium. Reflections are significant when there is a difference in acoustic impedance between the two media, known as an impedance mismatch.
This difference in impedance leads to part of the wave energy being reflected, with the amount of reflection increasing as the difference in impedance becomes greater.
The acoustic impedance Z for each medium can be represented with the formula Z = pu, where p represents the density (in kg/m³) and v (also represented as u in some contexts) represents the speed of sound (in m/s) through the medium.
The greater the difference between the acoustic impedances Z₁ and Z₂ of the two media, the more pronounced the reflection at their boundary.
So Option a is correct.