The blue nile river begins in lake tana. the lake is the river's
a. mouth
b. source
c. watershed
d. tributary

b. source

Explanation:

Lake Tana is located in the northern part of Eastern Africa. This lakes runoff actually produces a river, the Blue Nile River, thus it is the source of this river, as the river doesn't have a spring, but it starts from the lake itself.

Having a lake as a source has its advantages for a river, as its levels would be much more stable throughout the year, as the lake is not drying out.

The Blue Nile River is one of the two rivers that make up the Nile River, the other being the White Nile River. Once the Blue Nile and White Nile join in, they make up the longest river int he world, as well as one of the rivers with largest amount of water, the Nile River.

The Nile River is crucial for the whole region where it passes, as it is the basis for agriculture, only or one of the very few freshwater sources, and it is also extremely important for the industry, travel, and transportation.

a is the answer i believe.