The sentence from paragraph 2 that is least relevant to the claim "It is difficult to overstate just how popular YouTube currently is" is:
D. In some places outside of the U.S., the internet can, of course, be more difficult to access.
This sentence discusses internet accessibility in certain regions, which does not directly relate to the popularity of YouTube itself. The other options emphasize the platform's user base and its appeal across various age groups, directly supporting the statement about its popularity.