Degree of curve ($D$) can be defined either by arc definition ($20\text{ m}$ or $30\text{ m}$) or chord definition.

For a $20\text{ m}$ arc definition, $R = \frac{1145.92}{D} \approx \frac{1146}{D}\text{ m}$.

Deflection angle ($\Delta$) is equal to the total central angle subtended by the circular curve.

Simple linear proportion method allows quick field calculations without finding radius.

Applicable directly to both $20\text{ m}$ and $30\text{ m}$ arc definition conventions.

Assumes uniform curvature, which applies only to simple circular curves and not transition curves.

Alignment design of roads and railways for setting out simple circular curves.

Surveying calculations for curve layout using chainage and deflection angles.

Option A (320 m): Incorrect calculation resulting from using $D = 6.25°$ or wrong ratio.

Option B (480 m): Incorrect calculation resulting from using $30\text{ m}$ arc formula ($30 \times 100 / 5 = 600\text{ m}$) with an arithmetic error or wrong inputs.

Option C (600 m): Corresponds to a $30\text{ m}$ arc definition ($L = \frac{30 \times 100}{5} = 600\text{ m}$), but the question specifies a $20\text{ m}$ arc.