The method of finding out the difference in elevation between two points for eliminating the effect of curvature and refraction, is

ANS:A - reciprocal levelling

In reciprocal leveling.
If the level is placed exactly midway between two points and staff readings are taken to determine the difference of level then the errors (due to inclined colimiation line, curvature and refraction. ) are automatically eliminated. Answer is 3 because in reciprocal levelling curvature of the earth is eliminated and refraction is considered while differential levelling both are eliminated.