What are tributaries?

A. streams that flow into oceans

B. Rivers that flow into the other streams

C. Rivers that flow into lakes

D. Ocean currents that flow into other oceans

Tributaries are streams or rivers that flow into a larger main river. This much larger river also is referred to as a main stem. A tributary also is called an affluent.

so I'm guessing Rivers that flow into lakes

C. Rivers that flow into lakes.