Here we need to consider the both the arc length i.e. $4\pi$ and $6\pi$

Since all the angles of equilateral triangle = 60°

Therefore
$\frac{{central angle}}{360}= \frac{{Arc length}}{circumference}$

$\frac{120}{360}= \frac{{4\pi}}{{2\pi*radius}}$

or $radius = 6$

or $diameter = 12$

Now if we consider arc length as $6\pi$ then,

$\frac{{central angle}}{360} = \frac{{6\pi}}{{2\pi*radius}}$

$\frac{120}{360}= \frac{{6\pi}}{{2\pi*radius}}$

or $radius = 9$

or $diameter = 18$

So only Option D satisfy