Actually the significance of tag is
How many blocks of main memory are mapped to same set of cache memory just like in case of direct mapping how many blocks of main memory gets mapped to a line(block) of cache memory..
So having said that ,
No of lines in cache given = 16
Associativity i.e. k = 4
So no of sets = No of lines in cache / k
= 16 / 4
= 4
But there are 64 blocks of main memory in total..
Hence no of blocks that will map to same set = No of blocks in main memory / No of sets
= 64 / 4
= 16
So no of tag bits as mentioned earlier the significance of tag = log216
= 4
Hence D) should be the correct option..