Bioaugmentation involves

The correct option is addition of microbes to a cleanup site.

Understanding Bioaugmentation:
Bioaugmentation is a specific biotechnology and bioremediation technique used to clean up contaminated environments, such as polluted soil or water. The process involves adding pre-grown, specialized microorganisms (such as bacteria, archaea, or fungi) to the contaminated site. These microbes are specifically selected or engineered for their ability to biodegrade and neutralize target hazardous pollutants, accelerating the natural cleanup process.

Evaluating the Options:
1. Eliminating sludge: While biodegradation processes can reduce organic waste, bioaugmentation is not defined simply as sludge elimination.
2. Plants usage for bioremediation: The use of plants to clean up environments is known as phytoremediation, not bioaugmentation.
3. Bioventing: This is an in-situ remediation technology that stimulates the natural biodegradation of pollutants by providing air or oxygen to existing soil microbes, rather than adding new microbes.
4. Addition of microbes to a cleanup site: This directly matches the definition of bioaugmentation, where exogenous microorganisms are introduced to augment the existing microbial population's degrading capabilities.