Every preprocessing directive must begin with:
a.
#undef
b.
#
c.
#define
d.
whitespace

B. #

Explanation:

Every preprocessing directive must begin with the # character.

For example:

#define : Used to define a macro

#ifndef : Conditional evaluation of macro

#include : Used to include other preprocessor header file as part of the code

As we can see each of the directives begins with the # character. This is a signal to the preprocessor to interpret the subsequent keyword as a directive.