Your answer would be when precipitation exceeds evapotranspiration.
Definitions:
Precipitation - any form of water which falls to the ground, like rain, snow, hail, etc.
Evapotranspiration - when water is transferred from the land to the atmosphere.
Now, let's look at the three logical choices. (when it rains more than it snows doesn't fit in nor make sense, as why would snowfall rate have to do with making rivers?)
They all have to do with relating the rate of rainfall and evaporation. For a river, we want more water on the ground, and less in the air. Therefore, we would want more precipitation and less evaporation.
This means your correct choice is the last choice.
Hope this helped!
