Final answer:
Assuming a conclusion without prior experimentation or observation is NOT a valid step in scientific investigation, which violates the principles of the scientific method.
Step-by-step explanation:
The step that is NOT a possible step of scientific investigation is (d) Assuming a conclusion without prior experimentation or observation. The scientific method is a systematic process that involves several key steps, including observing and collecting data, developing a hypothesis, and designing and conducting an experiment. These steps are critical in ensuring that scientific investigation remains objective and yields reliable results. Assuming a conclusion without experimentation or observation undermines the integrity of this process and is not a valid step in a scientific investigation.