Answer
$B$
Work Step by Step
$l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times 2 \pi r ~~~(a)$
or, $l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times \pi d~~(b)$
Here, $r$ represents radius and $d$ is diameter.
We are given that the arc measures $60^{\circ}$ and $r=12 \ cm$
Thus, equation (b) becomes:
$l=\dfrac{60^{\circ}}{360^{\circ}} \times 2\pi (12) =4\pi \ cm$
Thus, our correct option is $B$.
