A bevel protractor is a precision angular measuring instrument that combines a main (dial) scale graduated in degrees with a small vernier scale to read fractions of a degree. Just as a vernier caliper subdivides a linear scale, the bevel protractor uses the vernier principle to subdivide the angular main scale, allowing angles to be read far more finely than the 1° graduations alone would permit.

The least count (smallest angle that can be resolved) is derived from the mismatch between one main-scale division and one vernier-scale division:

• On the main scale, each division equals 1° = 60 minutes (60′).
• The vernier scale is constructed so that 12 vernier divisions coincide with 23 main-scale divisions (23°). Hence one vernier division = 23°/12 = 1° 55′.
• Least count = (value of 2 main divisions) − (value of 1 vernier division) = 2° − 1°55′ = 5′. Equivalently, LC = smallest main division ÷ number of vernier divisions expressed in minutes = 60′ ÷ 12 = 5 minutes (5′).

Therefore the instrument can distinguish angular changes as small as 5′, which is 1/12 of a degree — precise enough for toolroom and inspection work.

Why the other choices are wrong: reading only to the nearest 1° ignores the vernier entirely and describes a plain protractor, not a vernier bevel protractor. A value of 0.1° (6′) does not match the standard 12-division vernier construction. A resolution of 10 minutes would correspond to a coarser 6-division vernier, which is not the conventional design; the widely accepted standard least count is 5 minutes.