time is the dependent variable, so the function normally goes:
F(t)..i.e. it is a function of time.
This function takes the length of the race, and returns the time needed to complete it. So the independent variable is the length of the race. The two real options here are B. Length(time) and Time (length).
Now the question says it takes the length of the race , lets call this Length(time).
Of course length is related to time, just think of your watch. So my answer is B Length (Time)