The correct answer is C. Gradients.
A gradient is a gradual transition between two or more colors. CSS3 gradients let you display smooth transitions between two or more specified colors. You can use gradients to create a variety of effects, such as a background that fades from one color to another, or a text that changes color as it scrolls down the page.
Here is an example of a CSS3 gradient:
background-image: linear-gradient(to right, #f00, #00f);
This gradient will create a background that fades from red to blue. The first color, #f00, is the starting color, and the second color, #00f, is the ending color. The direction of the gradient is specified by the to right keyword.
You can also use gradients to create radial gradients, which are gradients that radiate from a central point. To create a radial gradient, you use the radial-gradient() function. For example, the following code will create a radial gradient that fades from red to blue:
background-image: radial-gradient(circle at center, #f00, #00f);
The circle at center keyword specifies that the gradient should radiate from the center of the element.
CSS3 gradients are a powerful tool that can be used to create a variety of effects. They are easy to use and can be applied to any element on a web page.