In tacheometric survey, the additive constant is given by:
(Where, f - focal length of objective d - distance between optical centre and vertical axis of instrument)

Correct option is C

In tacheometric surveying, the additive constant (C) accounts for the distance from the instrument’s vertical axis to the optical center plus the focal length of the objective.
Mathematically, it is given by:
C = f + d
where
f = focal length of the objective lens
d = distance between the optical center of the objective and the vertical axis of the instrument