The key to a successful scientific investigation is A) developing a correct hypothesis B) developing a variety of hypotheses C) a complex experiment D) including two or more controls in the experiment E) making careful observations

E - Making careful observations.

Explanation:

The key to a successful scientific investigation is making careful observations given that, during an experiment, this observations could provide with evidence, premises, hypothesis (at later stages of the scientific process) and the possibility of conducting two or more controls in the experiment. Observation would allow to develop several hypothesis, for example, if the year was the 200 BC, and we wanted to find out why an apple falls to the ground when we throw it we could, after making careful observations that: %100 percent of the times we drop something it falls to a place, higher or lower, but always in the direction of the Earth. Therefore, we could reason that something draws or pushes everything that way. We could assume it's a god, or nature herself, develop many experiments and hypotesis, etc etc. All of this out of simple and careful observation of the facts.