B. visual learners. Having graphics is having something for learners to visually see, so having graphics is helpful for those who are visual learners, or people who learn better by seeing what is being taught, rather than just having someone tell them something.
A visual learner is someone who retains and absorbs information better through visual graphics and pictures. For example, if someone who is a visual learner is looking at a powerpoint packed with pictures and taking notes, they'll learn easier as opposed to if they learned by taking the textbook and copying notes from the words.
There are other types of learners too, like kinesthetic learners, but the majority of those (two-thirds as it states here in this problem) are visual learners and therefore graphics are ultimately more effective if presenting to an audience.
Since 1913, the medical knowledge has vastly changed. Medical terms that are today common-knowledge (DNA, chemotherapy, etc.) were not even in existence in 1913.
