A brief summary of the initial problem, what you have done and your most important results.
Answers to your sub-questions and then your main questions (using the results)
The limitations of your research.
Every research has limitations, because of limited time, people, or other resources, or sometimes simply because you made a mistake or did not foresee something. Be open about these limitations of your work, so that others can take them into account when making decisions based on your report (Would you bet your smartphone (or: your reputation) on the answer you provide here?)
Recommendations for the project / product / people you did this research for, and for any follow-up research (based on the results, conclusions and limitations of the research).