The correct answer is $\boxed{\text{B. 30°}}$.
A roof truss is a structural framework that supports the roof of a building. It is made up of a series of triangular frames that are connected by horizontal and vertical members. The angle of the roof truss is the angle between the top and bottom chords of the truss.
The angle of the roof truss is important because it affects the amount of snow and wind that the roof can withstand. A steeper roof will be able to withstand more snow and wind than a shallower roof. However, a steeper roof will also be more expensive to build.
The minimum angle of a roof truss with asbestos sheets is 30°. This is because asbestos sheets are not as strong as other types of roofing materials, such as shingles or metal roofing. If the angle of the roof truss is less than 30°, the asbestos sheets may not be able to withstand the weight of the snow and wind.
The other options are incorrect because they are not the minimum angle of a roof truss with asbestos sheets.