Which of the following is not a browser-specific versions of linear-gradient()?

The correct answer is C. -mac-linear-gradient().

The other options, A, B, and D, are all browser-specific versions of linear-gradient().

• -moz-linear-gradient() is a Firefox-specific version of linear-gradient().
• -o-linear-gradient() is an Opera-specific version of linear-gradient().
• -webkit-linear-gradient() is a WebKit-based browser (Chrome, Safari, etc.) version of linear-gradient().

The -mac-linear-gradient() keyword is not a valid CSS keyword.