Final answer:
Observation is the simplest method for gathering primary information out of the options provided, but the method chosen often depends on the research question, with interviewing serving as an important primary research technique for acquiring in-depth qualitative data.
Step-by-step explanation:
The simplest approach to gathering primary information among the listed methods is observation. Observation allows researchers to collect data in the subject's natural environment without manipulation or interference. It can be a passive process where the researcher simply watches the behavior in a specific setting. However, each research method including observation, focus groups, experimentation, and interviewing has its unique strengths and is selected based on the research question at hand. Interviewing involves engaging in a dialogue with someone who has knowledge about the research topic. This can provide in-depth data and a wealth of qualitative information. When it comes to primary research, interviewing can be a crucial technique for acquiring firsthand narratives and opinions directly from the source.