ITN $($Version $7\mathrm{.}00)-$ IP Addressing Exam

$5$ What is the prefix for the host address $2001$:DB$8$:BC$15$:A:$12$AB::$1/64?$

$2001$:DB$8$:BC$15$

$2001$:DB$8$:BC$15$:A

$2001$:DB$8$:BC$15$:A:$1$

$2001$:DB$8$:BC$15$:A:$12$