In a circle of radius 21 cm, an arc subtends an angle of 60° at the centre.

We have $r=21 \mathrm{~cm}$ and $\theta=60^{\circ}$

Length of $\operatorname{arc}=\frac{\theta}{360^{\circ}} \times 2 \pi r=\frac{60^{\circ}}{360^{\circ}} \times 2 \times \frac{22}{7} \times 21=22 \mathrm{~cm}$

Hence, the correct answer is option (b)