Option B: 1 : 4
Arc length = $\dfrac{90}{360} \times 2\pi r = \dfrac{1}{4} \times 2\pi r$. Ratio of arc length to circumference $= \dfrac{\frac{1}{4} \times 2\pi r}{2\pi r} = \dfrac{1}{4}$, i.e., 1 : 4.
The arc length formula $\dfrac{\theta}{360} \times 2\pi r$ gives the arc as a fraction of the full circumference $2\pi r$. With $\theta = 90°$, that fraction is $\frac{90}{360} = \frac{1}{4}$, directly giving the ratio 1 : 4. Always cancel $2\pi r$ to find the ratio quickly.