it is clear that it cant be 21 bits or 20 bits because in these cases the cache size will be nearly equal to main memory. so remaining option are 11 bits and 16 bits. here i will go with 16 bits because in case of 11 bits if memory is byte addressable as we know 5 bits will be wasted and also the memory size should be in power of 2. i already have checked the answer in answer key it is 16 bits.