Add Markdown Hightlight feature to Pluma, MD isn't supported by gtksourceview-2.0, so we cannot see the option for MD for Pluma. So, the support for MD is in gtksourceview-3.0. So simply to make a symbolic link into gtksourceview-2.9 is the way to fix or work around this issue.
Check libgtksourceview in the system,
If no, install it via
Go gtksourceview-2.0, and make an link from gtksourceview-3.0, and restart your Pluma.