How can a CSS variable be defined?

A CSS variable is defined using the syntax that consists of two hyphens followed by the variable name, which is represented as --variableName. This convention allows you to create custom properties in CSS that can be reused throughout your stylesheet. CSS variables are particularly useful for theming and maintaining consistency, as changes to the variable will automatically reflect wherever it is used.

When you declare a CSS variable, it is typically done within a selector, and you can then use the variable with the var() function wherever you need it, for example, color: var(--variableName);. This flexibility makes CSS variables powerful tools in modern web design.

The other choices do not adhere to the CSS variable syntax, which is why they are not valid for defining CSS variables.