The correct answer is C. border-radius.
The border-radius property is used to round the corners of an element. It takes a value of one or more numbers, each of which represents the radius of a corner. The numbers can be specified in different units, such as pixels, percentages, or ems.
For example, the following CSS code will round the corners of an element with a radius of 5px:
border-radius: 5px;
The following CSS code will round the corners of an element with a radius of 5%:
border-radius: 5%;
The following CSS code will round the corners of an element with a radius of 5em:
border-radius: 5em;
The border-radius property can also be used to create rounded corners on an element with a border. For example, the following CSS code will create a rounded rectangle with a border of 1px:
border: 1px solid black;
border-radius: 5px;
The border-radius property is a very versatile property that can be used to create a variety of different effects. It is a common property used in web design and development.