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
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