Which of the following would be used to create a class called button which has a width of 750px, a height of 30px and the color of the text is black?

The correct choice for creating a class called button with the specified properties is accurately described in the third option. When defining styles in CSS for a class, the syntax begins with a period (.) followed by the class name. In this case, .button correctly indicates a class selector.

The defined properties are essential for styling: height and width specify the dimensions of the button as requested, while the color property defines the text color. Using "color: black;" is the appropriate way to set the text color in CSS, which aligns with the requirement stated in the question.

This option captures all required elements correctly, including proper use of both the class symbol and the specific CSS properties necessary for the styling of the button. Other options either use the wrong selector or improperly specify properties, which would not achieve the intended outcome.