The problem at hand involves understanding the distribution of stress in the context of pure bending, which is a fundamental concept in the subject of Strength of Materials.

In pure bending, it is assumed that:

• The beam is initially straight, and its material is homogeneous and isotropic.
• Cross-sections that were plane and perpendicular to the longitudinal axis of the beam before bending remain plane and perpendicular after bending.
• There are no transverse forces acting, hence bending is symmetric.

Given these assumptions, we need to determine the stress distribution across the section of the beam.

Concept Explanation: In the theory of bending, particularly under the assumption of pure bending, it is asserted that the plane sections before bending remain plane after bending. This simplification leads to a theoretical basis for analyzing internal stresses.

The stress distribution across the depth of a beam under pure bending is described by the bending stress formula:

\(\sigma = \frac{My}{I}\)

• \(\sigma\) is the bending stress at a distance \(y\) from the neutral axis.
• \(M\) is the bending moment.
• \(I\) is the moment of inertia of the beam's cross-sectional area about the neutral axis.
• \(y\) is the perpendicular distance from the neutral axis to the point where the stress is being calculated.

The bending stress varies linearly with the distance from the neutral axis \(y\), thus confirming a Linear stress variation across the section.

Conclusion: Considering the given options, the correct answer is:

This option correctly describes the stress distribution resulting from the assumption that plane sections remain plane after bending.