Introduction:
The self-cleansing capacity of a river refers to its ability to purify itself naturally by removing pollutants and maintaining a healthy ecosystem. Various factors can affect this capacity, including aquatic organisms, water withdrawal for irrigation, hydroelectricity generation, and pollution. Among these factors, pollution has the most significant impact on the self-cleansing capacity of a river.

Pollution:
Pollution from various sources such as industrial discharge, agricultural runoff, and domestic sewage can severely degrade the water quality of a river. When pollutants enter the river, they can disrupt the natural balance of the ecosystem and harm aquatic organisms. Pollutants can include harmful chemicals, heavy metals, excess nutrients like nitrogen and phosphorus, and organic matter.

Impact on Aquatic Organisms:
Pollution adversely affects aquatic organisms, which are an essential component of the river ecosystem. Excess nutrients in the water can lead to excessive algae growth, causing oxygen depletion through the process of eutrophication. This oxygen depletion can result in the death of fish and other organisms that rely on oxygen for survival. Additionally, toxic chemicals and heavy metals can accumulate in the tissues of aquatic organisms, leading to health problems and reduced population sizes.

Water Withdrawal for Irrigation:
While water withdrawal for irrigation can impact the overall water availability in a river, it does not directly affect the self-cleansing capacity of the river. However, excessive water withdrawal can lead to reduced flow rates, which can in turn affect the dilution and dispersion of pollutants. This can exacerbate the pollution levels in the river, further hindering its self-cleansing capacity.

Hydroelectricity Generation:
Similar to water withdrawal for irrigation, hydroelectricity generation does not directly affect the self-cleansing capacity of the river. However, the construction of dams for hydroelectricity generation can alter the natural flow patterns of the river and disrupt the habitat of aquatic organisms. This can indirectly impact the self-cleansing capacity by reducing biodiversity and altering ecological processes.

Conclusion:
While factors such as aquatic organisms, water withdrawal for irrigation, and hydroelectricity generation can have indirect impacts on the self-cleansing capacity of a river, pollution is the primary factor that directly affects the river's ability to purify itself. Therefore, the correct answer is 'D' - Pollution. Efforts should be made to reduce pollution and improve water quality to maintain the self-cleansing capacity of rivers and ensure the health of their ecosystems.