What type of gradient would a river have when it is actively eroding its stream bed?

High gradient

Explanation:

The river bed of a stream is eroded when the local water velocity at the river is high. This occurs when the river gradient is high (meaning it slopes steeply) usualy higher up the river close to the watershed. This is different from when the river has low gradient which results in low water speeds. In this case, sediments in the river settle and erosion mostly happen on the river banks.  

a and b because with the other three, they simplify to something else.