What is the most important reason to create source cards?
they include an image or graphic that you wish to include in your paper.
they include specific information from a source for inclusion in your paper.
they include a list of possible sources of information about your topic.
they include the information you will need to reference your material or locate it again.

THe answer is:

They include the information you will need to reference your material or locate it again.

A source card has the information of the source that you used to make a reference or give facts or quotations, in the source card you can search for the information of the author, the publication, the location of the source, the title of the document, all things that will make it easier for you to find that source again, or to make proper citations.

i think the correct answer is b. phrase