5 + 6 + 6.
no. of words in 1 block = 256 B / 4 B = 64 => we need 6 bits for representing words i.e. offset.
cache lines = cache size/block size = 16 KB / 256 B = 64 => we need 6 bits to represent cache lines.
memory is of 128 KB => 17 bits for memory
bits for tag = 17 -6 -6 = 17 -12 = 5.