It's not. (EDIT: it is required to be at least 8 bits though)
The C standard specifies so much less than everyone thinks.
Common misconception.
to be at least 8 bits though
At least 8 bits for the latter-day compilers for which the 'standard' was the guide. I have seen C compilers where char was 6-bit or 7-bits matching the underlying hardware. However, those probably never implemented more than K&R or some pre-standardization perversion of it.
Also...as noted a char can be more than 8 bits, pre- and post-standardization. I've heard tell of (at least) 9-, 16-, 24- and 32-bit chars on more or less weird platforms.
At some point I'm sure the standards bodies will do some variation of depreciating 8-ish-bit char's for 32-bit Unicode as the standard representation for text.
I thought char was required by the C standard to be 8 bits. If everything is 24 bits, how do you cover 24 bit address space with 8 bit pointers?