Which CSS3 property allows you to change property values smoothly over a given duration?

The transition property in CSS3 is designed to facilitate smooth changes to property values over a specified duration. This property allows developers to define how certain CSS properties should change when their values change.

For example, if you want to change the background color of a button when a user hovers over it, you can use the transition property to create a gradual change from the original color to the new color. This results in a visually appealing effect rather than an abrupt change, improving the user experience.

In using the transition property, you specify the CSS property to be transitioned, the duration of the transition, and optionally, the timing function and delay. This creates a controlled and smooth transition effect.

The other options do not enable the feature of changing property values over time. Focus and hover are pseudo-classes that apply styles when elements are in certain states, but they do not inherently facilitate smooth transitions. Change is not a CSS property but rather a general term. Therefore, the transition property is the correct choice for creating smooth animations in CSS.