In designing reinforced concrete structures, a bent-up bar is used to resist negative bending moments. A positive bending moment typically occurs in the central span of a beam, while a negative bending moment is developed over the supports.

To effectively resist these moments, bars are bent up at points where negative moments are expected, usually close to the supports. This allows the reinforcement to actively engage in resisting the anticipated forces.

Let's analyze the given options:

1. Option (a): This diagram shows a straight bar without any bends. Therefore, it is not suited for resisting negative bending moments.
2. Option (b): This diagram shows a bar that is bent upwards in two places. However, the bends are located symmetrically in the center, which is unusual for typical support conditions and is more consistent with a different loading pattern.
3. Option (c): **Correct Answer**. This diagram shows a bar bent upwards towards both ends, which is characteristic of how negative bending moments are resisted over supports in continuous spans or certain types of cantilever structures.

Option (c) is the correct answer as it illustrates a bar designed to counteract negative bending moments effectively. The bends help develop the required tension forces where bending is detrimental and need to be resisted actively.