What Does the Question Mark Character ('?') Mean?

Additionally to other answers, ? can be part of a trigraph.

This is a ternary Operator which is conditional operator uses like if-else

example

int i=1;
int j=2;
int k;
k= i > j ? i : j;
//which is same as
if(i>j)
  k=i;
else
  k=j;

Usage: Syntax of ?: is

assignment_Variable = Condition ? value_if_true : value_if_false;

That’s probably a part of the ternary operator:

const int numApples = …;
printf("I have %i apple%s.\n", numApples == 1 ? "" : "s");

This is a so called conditional operator. You can shorten your if else statement with this operator.

The following link should explain everything

It is a conditional operator. For example refer the below link

Its the ternary operator, see

Most likely the '?' is the ternary operator. Its grammar is:

RESULT = (COND) ? (STATEMEN IF TRUE) : (STATEMENT IF FALSE)

It is a nice shorthand for the typical if-else statement:

if (COND) {
    RESULT = (STATEMENT IF TRUE);
} else {
    RESULT = (STATEMENT IF FALSE);

as it can usually fit on one line and can improve readability.

Some answers here refer to a trigraph, which is relevant to the C preprocessor. Take a look at this really dumb program, trigraphs.c:

# /* preprocessor will remove single hash symbols and this comment */
int main()
{
    char *t = "??=";
    char *p = "??/"";
    char *s = "??'";
    ??(, ??), ??! ??<, ??>, ??-
    return 0;
}

invoking only the c preprocessor by running gcc -E -trigraphs trigraph.c the output is

int main()
{
 char *t = "#"
 char *p = "\"";
 char *s = "^";
 [, ], | {, }, ~
 return 0;
}

Hopefully that clarifies a little bit what a trigraphs are, and what a '?' "means" in C.