a)
You got to think in the two variables you got in each case.
It´s really easy because they actually are the answers. Then determinate if one of them changes if the another change, if it does, it is the dependent variable.
How the car's millage is changing and it affects the car's price, it is the dependent variable.
So the right answer is A.
b) Same logic.
How the amount of sleep is been afected by the amount of medicament, it means that the amount of medicament is the independent variable. So the dependent variable is the amount of sleep.
The correct answer B
c) Same logic but with letters,
In the ecuation if you change the value of k, it will change the value of n. It means that the first one is the independent variable and the second one is the dependetn.
So the correct answer is B.