The answer for the question below is 3.
Why can't 4 be a valid unicast address?
Which address is a valid IPv6 unicast address?
Taken the same example as Ganesh.
this example shows how You can group a set of zeroe's together and write it as :: instead.
However this double :: can only be present once, so an address written as
2001:cdba:0:0:1:0:0:9652 can be written either as
2001:cdba::1:0:0:9652 but not
In question 1,2 and 4 there are two groups of :: , this is not valid, and in q5 it's written ::::, wich is also not a legal address.