What is The IPv$6$ loopback address?

$127\mathrm{.}0\mathrm{.}0\mathrm{.}1$

$0000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0000$

$1000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0000$

$0000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0000$:$0001$