Final answer:
Diaries of European travelers would provide the best primary sources for examining the impact of the printing press and printed books on Europe, as they contain first-hand accounts and insights into societal changes, humanism, and the challenges to authority.
Step-by-step explanation:
The best primary source to examine how the printing press and printed books influenced Europe would be diaries of European travelers (option a). These diaries would provide first-hand accounts of the changes and impacts seen in society due to the spread of printed materials. They would offer insights into the dissemination of knowledge, the rise of humanism, and the challenges to religious and political authority, as influenced by the proliferation of printed works.
For instance, the impact of printed books was evident through the works of individuals like Martin Luther, whose reformist texts significantly altered European thought and Christianity. As such, a diary from this period could reveal the immediate reactions to and reflections on these developments. Furthermore, it's important to understand that primary sources must be used with caution considering potential errors or changes in transmission, as well as the presence of sensationalism and censorship in early print media.