This issue came up recently for me (and I've been prompted to lodge it as a specific question about it):
My system is not dual-boot, I run a standard Ubuntu desktop system "on the metal" (I think running it in a VM is the same).
Pressing c while booting does not cause the GRUB menu to appear (which I believe it does for a dual-boot system).
How can I get the GRUB-menu to present itself on a single-boot system?