(a) 22 cm
Length of arc $= \dfrac{\theta}{360} \times 2\pi r = \dfrac{60}{360} \times 2 \times \dfrac{22}{7} \times 21 = \dfrac{1}{6} \times 132 = 22$ cm.
Use the formula: Length of arc $= \dfrac{\theta}{360} \times 2\pi r$. With $r = 21$ cm, $\theta = 60°$, and $\pi = \dfrac{22}{7}$, the calculation simplifies cleanly to 22 cm. Examiners award the mark for selecting (a) with or without the working shown, but showing one-line working is good practice.