In reciprocal levelling, the error which is not completely eliminated, is due to

ANS:C - refraction

Answer is C because in question the said to completely eliminate error. Means due to refraction the error eliminated but partially not completely. And options C is not related to eliminate partially or completely. Error due to earth curvature is partially eliminated other boths are fully eliminated. Actually, we assume that refraction is same at both points but the fact is, refraction is a function of temperature and pressure which we can't control all other errors can be eliminated but the error due to refraction can't. The elimination of the error due to refraction depends upon the change in the climatic conditions during the transfer of the instrument hence the error due to refraction may not be removed completely.

Ref: R. Agor.