What primarily determines sonic layer depth?

The correct choice is based on the principle that the sonic layer depth is significantly influenced by sound velocity, which is determined by various physical properties of seawater, primarily temperature, salinity, and pressure. The sound velocity increases with temperature and salinity while it decreases with depth due to increasing pressure.

The concept of the sonic layer specifically refers to a layer in the ocean where sound waves travel at a particular speed, and this speed can create layers where sound propagates differently due to changes in water properties with depth. Therefore, the cumulative effect of these factors results in distinct sound speed profiles that define the sonic layer depth.

While temperature, pressure, and salinity each affect sound velocity, it is the sound velocity profile derived from these factors that ultimately determines where the sonic layer exists within a body of water. Hence, sound velocity is the primary determinant of sonic layer depth, making it the most appropriate response to the question.