For an external focusing telescope, the additive constant $C$ ranges from $30\text{ cm}$ to $45\text{ cm}$ ($0.30\text{ m}$ to $0.45\text{ m}$).

For an internal focusing telescope, the additive constant $C$ ranges from $8\text{ cm}$ to $15\text{ cm}$ ($0.08\text{ m}$ to $0.15\text{ m}$).

When an anallatic lens is fitted in a tacheometer telescope, the additive constant $C$ becomes exactly zero ($C = 0$).

Fixed tube length prevents dust and moisture entry into the telescope barrel.

Small additive constant reduces instrumental error in stadia measurement calculations.

Slight loss of illumination due to reflection and absorption at the additional internal lens surfaces.

Internal mechanism makes lens cleaning and alignment adjustment more complex.

Determining horizontal distance $D$ directly without physical chaining in rough terrain.

Preparation of topographic maps and contouring in steep or inaccessible areas.

Option A (0 cm to 5 cm) is incorrect as zero additive constant is achieved specifically using an anallatic lens, not standard internal focusing.

Option B (5 cm to 10 cm) and Option D (15 cm to 20 cm) are outside the standard empirical specification range of 8 cm to 15 cm for internal focusing tacheometers.