Which of the following CSS3 Color Feature like RGB color but adds an alpha chann

Which of the following CSS3 Color Feature like RGB color but adds an alpha channel value to specify the opacity of the color?

2) Which of the following Color Format is a CSS3 HSL value with a fourth value to set the alpha channel value for the color to define the opacity of the element?

3) Which of the following CSS3 Color Feature like RGB color but adds an alpha channel value to specify the opacity of the color?

4) Which of the following Color Format are a 17 defined colors under CSS 2.1?

5) Which of the following Color Format is a CSS2 introduced named color keywords which allows Web page colors to be matched to an operating system’s color use?