Answer- C. Horizontal hairs
The diaphragm of a stadia theodolite is fitted with two additional horizontal hairs
Stadia line or the stadia hairs is the crosshairs marked on the reticle of a theodolite.
Stadia hairs are the horizontal hairs marked in the theodolite are in any other surveying instrument.
Intercept between stadia hairs is known as stadia interval denoted as “s”.
The horizontal distance can be calculated as D = Ks+C
- where K is multiplying constant and C is the additive constant.