Q: Can I install the Active Directory Rights Management Service role on my domain controller?
A. Although it's possible to install the Active Directory (AD) Rights Management Service (RMS) role on a domain controller (DC), this isn't recommended because RMS requires very specific permissions on the RMS service account. These permissions include local administrator rights, which on a DC would mean domain privileges-- a poor security practice.
The best practice is to run RMS on a dedicated OS instance, which, of course, can be a virtual machine (VM). For some other best practices, see Microsoft's website.