Which of the following is a crucial reason water affects slope stability?

Water plays a significant role in influencing slope stability, and one of the critical factors is its contribution to the overall weight on slopes. When rainwater infiltrates the soil and other materials on a slope, it adds extra mass. This increased weight can exceed the slope's ability to maintain stability, particularly if the slope is already at risk of failing.

Additionally, as water saturates the soil, it may lead to a reduction in frictional forces that help keep the slope intact. This combination of added weight and reduced friction increases the likelihood of landslides or slope failure.

In contrast, while vegetation growth can help stabilize slopes due to root systems, it does not directly contribute to the immediate effects of weight and pressure. Similarly, stagnant water on slopes typically can lead to other problems but does not enhance stability. Cohesiveness due to water is a more complex interaction that can vary depending on soil type but is generally not as critical as the added weight that water contributes. Therefore, the increase in weight caused by water accumulation is the most direct and clear reason for its impact on slope stability.