Answer:
d) Make it flexible
Step-by-step explanation:
Raw hides dry out to form a hard material that can decay with an offensive smell when they become wet.
Tanning converts the protein of the raw hide into a stable material that will stay flexible and not decompose when it gets wet.
a) is wrong. The purpose of tanning is to soften the leather.
b) is wrong. You smooth the leather manually with fine sandpaper.
c) is wrong. Tanning does not impart water resistance. It prevents the leather from putrefying when it does get wet.