The correct option is "d. All of the above"
Visual aids are means used in public speaking to "visually" express a concept or an idea and serve to stimulate the imagination of the listeners or to highlight a basic idea in our discourse. It is scientifically proven that visual communications are synthetic messages that speak to the eyes and are more effective than words. These visual aids can be: the blackboard, the flip chart, the overhead projector, the slides, the films, the maps, the diagrams, the plans, the drawings and even the scale models. They complement verbal forms and contribute to presenting the topic in a clearer and more convincing way.