Final answer:
Visual aids should be used to illustrate ideas that are difficult to convey strictly with words. Slides should have minimal text and use clear and concise visuals.
Step-by-step explanation:
The correct statement regarding the use of visual aids in oral presentations is (a) Visual aids should be used to illustrate ideas that are difficult to convey strictly with words. Visual aids such as images, graphs, and charts can enhance the audience's understanding of complex ideas and make them more engaging.
On the other hand, statement (b) which suggests allowing at least six minutes of spoken presentation for each slide is not necessarily a correct approach. The amount of time spent on each slide should depend on the content and the flow of the presentation to maintain the audience's attention.
Statement (c) that slides should be colorful with substantial amounts of text is not correct. Slides should have minimal text to avoid overwhelming the audience and should use clear and concise visuals to support the presenter's message.