Base level is

a. the source from which the stream flows
b. the lowest point to which a stream can erode its channel
c. any part of the stream that is below sea level
d. the elevation of a stream’s largest tributary

Optiion C.

The lowest point in which a stream can erode it's channel

Explanation:

Base level is the lowest level in which a stream can erode it's channel.

The stream of water stop flowing when it reaches it's base level. The stream of water cannot cut deeper into it's base so it's cut into banks side ways. The stream then meanders and widen it's valley as a result of cutting into it's banks

We have two types of base level, the ultimate and local base level.

The local base level is the point at which the river can erode it's channel locally.

from the original order (top to bottom) the new order will go

1 (founder)

4 (hawk)

2 (mutate)

3 (next generation)