Final answer:
The statement "the activity of hair cells in the vertebrate inner ear depends on a high concentration of extracellular K+" is true because the hair cells in the inner ear are responsible for converting sound vibrations into electrical signals that the brain can interpret. The correct option is a.
Step-by-step explanation:
This conversion process, known as transduction, depends on the presence of extracellular potassium ions (K+). When sound vibrations cause the stereocilia on the hair cells to bend, it opens gated ion channels. This allows an influx of potassium ions into the hair cells, depolarising the hair cell membrane and generating electrical signals.
Hence, a is the correct option.