A. the Readers' Guide is a basic index, while the other resources are full databases allowing access to actual articles.
An index is an alphabetical list of items. In this case, it is a list of all the periodicals published and is sorted by subject. One would use this source if it wanted to know which periodicals are written on the topic of education or psychology. EBSCOhost and Expanded Academic ASAP are full databases that contain the articles within the periodicals. They are easily searchable using keywords or other advanced search conditions.