How is the animation 'increase-font' defined to increase font size from 5px to 20px?

The correct answer defines the animation using the proper syntax for creating keyframes in CSS. By using the @keyframes rule, the animation is set to transition between specific styles at distinct points in time. In this case, the keyframe animation named 'increase-font' specifies that at the beginning of the animation (the from state), the font size should be 5 pixels. At the end of the animation (the to state), the font size transitions to 20 pixels.

This approach is standard and widely recognized in CSS for specifying animations, which makes it effective for creating smooth transitions in web design. The use of from and to in the syntax directly indicates how the animation progresses from the initial state to the final state, allowing browsers to understand how to animate the property changes over time.

In contrast, the other answers utilize incorrect syntax, such as using start and end keywords, or using @animation, which is not a valid CSS at-rule for defining animations. These options would not function correctly in typical web design scenarios.