Aresearcher investigated the relationship between test length and grades in a western civilization course. after randomly assigning students to groups, she found that students who took longer exams received better grades than students who took shorter exams. the independent variable was

I believe the answer is: Length of the test

In a research, independent variable refers to the variables that would influence the data recollection from other variables.
The variation of independent variable is not dependent on other variable and could freely changed by the researchers to see if it's making different outcome.