To solve this question, the user needs to know about the different PHP tags and their compatibility with XML or XHTML.
Option A: ?> is a valid PHP tag but it is not recommended to use as it may conflict with XML processing instruction. It is also not recommended to use with XHTML.
Option B: is not a valid PHP tag and will not be parsed as PHP code. It may work on some servers with specific configurations, but it is not portable or recommended.
Option C: is a valid PHP tag, but it is not compatible with XML or XHTML. It is used in ASP-style syntax.
Option D: is the recommended and most portable version of the PHP tag. It is compatible with XML or XHTML and is widely supported on different servers and configurations.