Why do '?' and '\?' give the same output in C?

In C, why do these two pieces of code give the same output?

#include<stdio.h>

int main(void)
{
    const char c='\?';
    printf("%c",c);
}

and

#include<stdio.h>

int main(void)
{
    const char c='?';
    printf("%c",c);
}

I understand that a backslash is used to make quotes (" or ') and a backslash obvious to the compiler when we use printf(), but why does this work for the '?'?

\? is an escape sequence exactly equivalent to ?, and is used to escape trigraphs:

#include <stdio.h>
int main(void) {
    printf("%s %s", "??=", "?\?="); // output is # ??=
}

Quoting C11, chapter §6.4.4.4p4

The double-quote " and question-mark ? are representable either by themselves or by the escape sequences \" and \?, respectively, but ... .

_{Emphasis mine}

So the escape sequence \? is treated the same as ?.

Because '\?' is a valid escape code, and is equal to a question-mark.