In the case of a three-pinned parabolic arch carrying a uniformly distributed load on the entire span, then bending moment will be

ANS:D - zero throughout the span

It is because the structure is funicular with respect to the loading.

If shape of the bending moment as well as the structures geometry is similar in proportion, then the bending moment will be zero. The same happens with a triangular structure which has point load at the top of the vertex.

All those structures are funicular with respect to their loading. Similar concept is used in prestressed concrete cable profile (parabolic/linear variation along span).

The implication I get out of this is that, a flexible structure such as cable will experience negligible bending moments because the it can change its shape to become funicular with load.

However caution must be exercised, as a rigid structure such as concrete and steel will not be able to adapt its geometry to the loading. If designed for zero bending moment for a given loading (say for uniform Dead load and live load) the members or the structure as whole will experience moment under different loading condition say due to a point load or a lateral force due to wind or seismic actions.