Which of these human activities can increase the risks of flooding?
a. cultivation
b. planting trees
c. soil management
d. widening rivers

The best and most correct answer among the choices provided by your question is third option or letter C. Soil management can increase the risk of flooding. 
Soil management is an integral part of land management and may focus on differences in soil types and soil characteristics to define specific interventions that are aimed to enhance the soil quality for the land use selected.

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I think its d. i think it is d because if you widen the rivers then that makes space for more water.