High-frequency sounds primarily activate hair cells at the of the cochlea, whereas low-frequency sounds primarily activate hair cells at the of the cochlea.

The correct answer is "base; apex".

Explanation:

The cochlea is the region of the inner hear at which sounds are received and the sound waves start to be processed. The inner hair cells of the cochlea are responsible of transforming the sound vibrations. High-frequency sounds primarily activate hair cells at the base of the cochlea, the region that is narrowest and stiffest in the cochlea. On the other hand, low-frequency sounds  primarily activate hair cells at the apex of the cochlea, the region that is widest and least stiff in the cochlea.