we know that
A median of a triangle is a line segment joining a vertex to the midpoint of the opposing side, bisecting it.
In this problem the line DB is a median of triangle ADC
The line DB is also the altitude of a triangle ADC, because is perpendicular to the side AC
so
AB=BC
we have that
[tex]BC=9\ units[/tex]
therefore
[tex]AB=9\ units[/tex]
the answer is the option
[tex]9[/tex]
