Question

The response to or result of the experimenter's purpsoeful changes is the:
A. Independent variable
B. Control
C. Dependent variable
D. Constant

Answer 1

dependent variable is the one you control,
independent is the one you manipulate for reasons. so
( A.)

Answer 2

Dependent variable. That's why it's called dependent - It is dependent on what you do to the independent variable (as Hcgturtle has said).
Constant Variable is the one you want to keep exactly the same throughout the experiment. And a control is one you use to check your experiment. E.g. if you are testing effects of fertilisers on plants, you'd do a plant with no fertiliser as a control.