Which file is used by Id.so to find libraries quickly without actually having to search the directories in its library path?

Which file is used by Id.so to find libraries quickly without actually having to search the directories
in its library path?

A.
/etc/Id.so.cache

B.
/etc/Id.so.conf

C.
/etc/Id.so

D.
/var/ld/cache

E.
/var/.cache