Which type of source includes original documents like letters and speeches?

The correct answer is primary sources. Primary sources are defined as original materials that provide direct evidence or firsthand accounts of an event, phenomenon, or time period. These documents include letters, speeches, diaries, photographs, interviews, and original research findings, among others. The unique value of primary sources lies in their immediacy and authenticity, allowing researchers to engage directly with the material, gaining insights that secondary sources, which interpret or analyze primary data, cannot provide.

Secondary sources, on the other hand, analyze, interpret, or summarize the content from primary sources and do not constitute firsthand accounts. Peer-reviewed articles may include original research but are primarily considered secondary sources, as they discuss findings rather than present them in an original format. Reference sources such as encyclopedias or dictionaries provide summaries or overviews of information but do not contain original documents, making them unsuitable for this particular question.