It is as close to hardware as your userspace program is allowed to get. There is definitely a whole bunch of lower layers, but you cannot really get to them, s

It is as close to hardware as your userspace program is allowed to get. There is definitely a whole bunch of lower layers, but you cannot really get to them, so I think it's pretty fair to call C "a lowest layer language for userspace after assembly". And probably for kernel programming, too - C gets into implementation-defined behavior when there are MMUs involved, but that's not a big practical problem. (if you work at Intel and have access to secret microcode definitions than C is not a high level language... but cases like this are super rare)