Final answer:
The bottom section of a bending beam exclusively experiences tensile stress, which means it is subject to stretching forces.
Step-by-step explanation:
When a beam is subjected to bending, different parts of the beam experience different types of stresses. The top section of such a beam is placed under compressive stress, meaning it is being squeezed together. Conversely, the bottom section of the beam is placed under tensile stress, meaning it is being pulled apart. The middle of the beam, also known as the neutral axis, does not experience any tensile or compressive stress and remains undeformed.
Therefore, the correct answer to the question is b) Bottom, as the bottom part of the beam is the portion that exclusively experiences tensile stress when the beam is bending downwards under the weight it carries or its own weight.