The link for the source is: https://www.montana.edu/extension/ipm/cropweeds/projects/documents/ecology_and_management_of_canada_thistle-NRCS.pdf.
The source according to the information discussed in the tutorial is academic, peer-reviewed review material. The reason why I chose the above classification is because the paper was published by the United States Department of Agriculture, Natural Resources Conservation Service (NRCS) . It also has cited the sources from where the information presented has been reviewed and confirmed. Moreover the paper was written by researchers from an accredited university (Montana State University) and includes material for review and educational purposes only.