In my opinion, the main difference is D. the Readers' Guide is a basic index, while the other resources are full databases allowing access to actual articles.
The Readers' Guide indexes articles from over 450 periodicals from all the humanities, dating from 1983 onwards. You can find the information on a particular article there, as well as the topic it covers. But if you want to read the articles, you should look up another database, such as EBSCO Host or Expanded Academic ASAP.