In C$++$

Which of the following is the correct way to define a constant in $C++?$

a$)$ #define constant $P=3\mathrm{.}14$;

b$)$ #define PI$=3\mathrm{.}14$;

c$)$ #define PI $3\mathrm{.}14$

d$)$ #define constant$=3\mathrm{.}14$;