What controls how each slide replaces the previous one in a presentation?

Transitions are the effects that control how one slide replaces another in a presentation. They can create a visual effect or animation that occurs during the change from one slide to the next, enhancing the flow and overall impact of the presentation. Different types of transitions can give varying levels of emphasis and can help maintain the audience's attention, providing a smoother or more dynamic experience.

Animations, while related, define how individual elements on a slide appear or move, rather than the way one slide changes to another. Navigation refers to how the presenter or audience moves through the slides but does not directly affect the visual effect of slide changes. Slide timing dictates how long a slide is displayed before the transition occurs, but it does not influence the visual manner in which the slides transition from one to another. Thus, transitions specifically address the mechanics of replacing one slide with another, making them the correct answer to the question.