The best choice for conducting thorough research would be d. primary sources.
Primary sources provide direct or firsthand evidence about an event, object, person, or work of art. They include original materials such as interviews, documents, reports, photographs, artifacts, and direct observations.
While secondary sources (like those found in options a, b, and c) can offer valuable context and analysis, they interpret or analyze primary sources. Therefore, relying on primary sources is crucial for a more authentic understanding of the subject being researched.