The cache_dir directive in the Squid configuration file specifies the type, location, size, and structure of the cache directory. The ufs type indicates that Squid uses the UNIX file system to store the cache files. The /var/spool/squid3/ is the path to the cache directory. The 1024 is the size of the cache in megabytes. The 16 and 256 are the number of first-level and second-level subdirectories that Squid creates under the cache directory.

Squid uses hexadecimal numbers to name the subdirectories, from 00 to FF. Therefore, there will be 16 first-level subdirectories, named 00, 01, 02, …, 0E, 0F. Under each first-level subdirectory, there will be 256 second-level subdirectories, named 00, 01, 02, …, FE, FF. For example, the subdirectory /var/spool/squid3/0F/FF will contain the cache files whose hash values end with 0FFF.

Therefore, the correct answer is A. 0F and E. 00, as they are the names of the first-level subdirectories that will exist directly within the directory /var/spool/squid3/. The other options are either invalid or second-level subdirectories that will not exist directly within the directory /var/spool/squid3/.