This linked reference might help you.
Provides guidance to Office admins about Trusted Locations in Office apps.
learn.microsoft.com
Before I retired, I worked with the U.S. Navy and they were highly security-conscious about trust settings because of the implications of what you can do from trusted locations. (I'm avoiding "paranoid"...) It is possible depending on your environment to disallow that particular setting.
For a home computer or simple home network, domain policies will NOT be an issue so you should be able to get into the Trust Center to declare a trusted location. For a corporate environment, check with your IT Security folks before doing that.
The problem with declaring
C:\ (the root folder of C drive) as "trusted" is that if you DO get hacked, that is a common-place target for viruses, worms, root-kits, trojans, etc. AND if you make it trusted, they can do a lot more damage. On the other hand, 2nd-level folders of
C:\ should be no problem. Therefore, I would
recommend NOT making
C:\ trusted, but have no such recommendation for
C:\some-other-name\ to be trusted.
NOTE also that IF it happens that you have external drives mapped onto your system - such as a USB 3.0-connected hard drive with a terabyte or two of storage - that you might have mapping issues regarding the precise folder name. So if you mapped, let's say, a Q: drive to one of these external USB ports, the name to be trusted might not be
Q:\ (the drive-letter reference) but rather
\\system-name\drive-designation\ (the URS reference).