Which factor does not play a role in runoff
the amount of rain or snow
plants
gravity
solar radiation

Infiltration is the process by which rainwater becomes groundwater. In the water cycle, which involves primarily; evaporation, condensation and precipitation. Henceforth, after precipitation comes infiltration in the land areas leaving the rainwater aggregated in a single location which accumulates over time. Then this process is followed by runoff and subsurface flow by which water flows through other bodies of water, either in river, sea or ocean, even lake. Other rainwaters that has segregated and wasn’t able to transport itself stays in the process of infiltration, thus groundwater.

Thus, solar radiation doesn't play a role in runoff or infiltration.

Carbon-14 has a half-life of 5,730 years and most igneous rock is older than that.