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.

"the periodic table is arranged in order of atomic number."