ANS:C - 10 cm

When the surveys extended over limited areas the difference in length between the arc and the subtended chord for any two points on the earth's surface is only about 0.1m for a distance of 18.2 km, 0.3m for 54.3 km and 0.5 m for 91 km. For distance up to 18.2 km is 10 cm.
For distance up to 54.3 km is 30 cm.
For distance up to 91 km is 50 cm. For 12km difference is 10mm.
For 18km difference will be nearly 15mm.