Cache size is 256 KB .
4 way set associative cache means number of blocks per set is 4 .
Cache line size is 8 words( means each block size is 8 words ) and each word has 32 bits = 4 Bytes ,
so total Block size is 8 * 4 = 32 Bytes
32 = 25 so 5 bits require for block offset .
There are 4 blocks /set and each block size is 32 Bytes .
It is 4 way set associative cache , so number of sets are ( 256 KB/ 4 * 32 B )
= (28 * 210 / 22 * 25 ) =218/27
= 211
Hence 11 bits require to represent the index field .