Question

A watershed is _____.
an area where all water drains into the same river system
the start of a river system
the location where tributaries form rivers
a depression left by glaciers which forms a lake

Answer 1

Another word for "watershed" is drainage basin - it refers to all the land that will end in the same river and it's separated from land which will provide water for other rivers (but sometimes this separation is also called watershed- this can be confusing!)

so the correct answer is
an area where all water drains into the same river system

Answer 2

A watershed is an area where all water drains into the same river system.