Fully Associative Cache has fields: Tag, Offset.
Direct Mapped Cache has fields: Tag, Line No., Offset
Set associative cache has fields: Tag, Set No., Offset
As you can see, in fully associative cache, there's no index field. Hence index bits = $0$.
Now, Offset = $4$ bits.
=> Tag $= 32 - 4 = 28$ bits.
Option D