The question is about the permissible shear stress limits in a reinforced concrete beam as per IS 456:2000, a standard code of practice for plain and reinforced concrete in India. When the calculated nominal shear stress τ in a beam exceeds the maximum permissible shear stress τ_{cmax} provided by the standard, it indicates that the beam is experiencing more shear stress than it is designed to withstand. In such cases, specific actions are needed to ensure the safety and integrity of the structure.

Let's evaluate each option:

1. Use a lower grade of concrete to reduce stress: Using a lower grade of concrete would decrease the strength of the concrete, which is counterproductive when dealing with excessive shear stress. This option is incorrect because it would further compromise the structural integrity.
2. Increase the beam width only: Simply increasing the width of the beam may only have a limited impact on reducing nominal shear stress without addressing the overall structural design and may not suffice to counteract the already exceeded shear limits. This option is not the most appropriate by itself.
3. Provide additional shear reinforcement only: While additional shear reinforcement (stirrups or bars) can help to some extent, it is often not sufficient when the derived stress significantly exceeds permissible limits. Comprehensive redesign may still be necessary.
4. Increase the effective depth of the beam or redesign the section: Increasing the effective depth reduces shear stress because the lever arm of the beam is increased, leading to a lower moment of resistance. This option provides a more holistic approach to mitigating stress issues by modifying the structural design, making it the correct action.

Based on this analysis, the correct course of action when τ exceeds τ_{cmax} is to increase the effective depth of the beam or redesign the section. This involves recalculating the sectional properties to ensure compliance with IS 456:2000, ultimately ensuring the beam can handle the actual shear forces effectively.