Answer :
Final answer:
The most reliable source to research David Thompson's expeditions is an academic journal.
Explanation:
The most reliable source for researching David Thompson's pioneering expeditions would be an academic journal entry entitled, 'David Thompson (1770-1857)'. Such an entry is likely to provide detailed, well-researched information backed by scholarly expertise and primary sources. This contrasts with sources like a web-based slide deck or a column in a popular travel magazine, which may lack the depth and accuracy needed for academic research.
Learn more about reliability of sources for historical research here:
https://brainly.com/question/37054056