Which best describes the purpose of a control sample?

The purpose of having a control sample is the possibility of having something to compare the results of your experiment.

For example with medicine, the sample that received the medicine are in X state now, this has not enough information really, you need a control sample that is in a state Y, and now you can compare the states X and Y and see how your medicine really affects the patients.

Other example is how a substance X changes the color of something, if you do not have a control sample, at the end of the experiment you can't se the actual color change, so your really need a control sample.

The control sample is called the "zero" or starting point in an experiment.

kepler used the data from tycho blahe (his mentor) to construct the laws of planetary motion.