Well, the meaning of the 'D' in DC Comics is somewhat ambiguous. It could be a symbolic letter chosen for its aesthetic or branding purposes, or it might have a deeper meaning known only to the creators and insiders of the comic industry.
The 'D' in DC Comics is for Detective. It's a key part of the brand and is associated with the various detective-oriented stories and characters they've created over the years.
Well, 'DC' in 'DC Comics' represents Detective Comics. This was the original comic series from which the larger DC Comics universe expanded with various superheroes and storylines over time.
In DC Comics, 'DC' originally referred to Detective Comics. This was the comic where characters like Superman and others first appeared and became famous. Over time, 'DC' has come to represent the entire comic book universe and its various franchises.