The purpose of controlling an experiment is to

The answer is "D"! The reason is that if it is not controlled, the experiment would be useless.

The correct answer is D: MAKE SURE THE RESULTS OF THE EXPERIMENT ARE USEFUL.

Explanation:

When we are interested in making an experiment we should consider some things in order to obtain objective and relatable results that will be considered as scientific data. We usually make experiments as a part of an entire investigation that it is made to prove or refuse a HYPOTHESIS. At this respect we have to take into account the METHODOLOGY of the process, which is the part of an investigation who tell us what, how and for what are we going to experiment.

REMEMBER: Experimenting has to respond to general and specific objectives previously stated, WE CANNOT IMPROVISE! WE ALSO CANNOT DO FIRST THE EXPERIMENT AND THEN THE INVESTIGATION PROCESS!

As we have seen until here, making an experiment follows determined objectives, so controlling the factors that it involves will be depending on our objectives. By doing this we will be sure about its reliability and we will be concluding based on our results.

The other three options (A, B and C) are not applicable to an investigation process and this is why they are not correct.

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.

a:

by the combination of an unlimited number of elements