Final answer:
The distal end of the basilar membrane toward the apex of the cochlea is the most flexible, being thinner and floppier compared to the base which is tauter and responds best to high frequencies.
Step-by-step explanation:
The question asks which part of the basilar membrane is more flexible. The mechanical properties of the basilar membrane change along its length, with it being thicker, tauter, and narrower at the outside of the whorl and thinner, floppier, and broader toward the apex. The apex, or the center of the whorl, is where the cochlea is smallest and the basilar membrane is the most flexible.
This area is more susceptible to lower frequencies due to its flexibility. In contrast, the base of the basilar membrane, which is closer to the entrance port (the oval window), is less flexible and responds best to high frequencies. Therefore, to answer the question, the distal end of the basilar membrane, which is toward the apex of the cochlea, is the most flexible.