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 access issues in certain areas, which does not directly support the claim about YouTube's popularity. The other options provide specific evidence regarding YouTube’s user base and its appeal, which are more directly related to the claim of its popularity.