Which of these is not a valid IPv$6$ address that can be assigned to an interface?

Question options:

$2000$:$1$:$1$:$1$:$1$:$1$:$1$:$1$

$2000$:$1$:$1$:$1$:$1$:$2$

$2000$:$1$::$1$:$2$

$2000$::$2$

All are valid