Consider this scenario:
CPT: project
Page titled "Project" is being used to show a list/grid of all the projects rather than the CPT archive.
When viewing a single project, you want this Project Page's nav menu item to become active.
This Pro tutorial provides the code for using nav_menu_css_class WordPress filter to make the "Project" menu item active when viewing a single post of project custom post type.
Note that this can also be done using custom CSS but PHP solution is more solid.
Add the following in child theme's functions.php (w/o the opening PHP tag) or a code snippets plugin:
