Tk is written in Tcl, but the C API is a first class citizen, which is how you end up with things like Perl/Tk. And Tk is widely considered a breeze to work with, and (correct me if I'm wrong) may have been one of the first toolkits to handle widget nesting, placement, and resizing automagically.
So, it's a GUI toolkit for C which may be the easiest to use of all of them, even accounting for Qt, Silverlight, etc.