A production optimization problem has $4$ decision variables and resource $1$ limits hov many of the $4$ products can be produced. Which of the following constraints reflects fact?

f$($X$1,$ X$2,$ X$3,$ X$4)=$ b

b$.$ f$($X$1,$ X$2,$ X$3,$ X$4)>=$ b$1$

C$.$ f$($X$1,$ X$2,$ X$3,$ X$4)<=$ bi

d$.$ f$($X$1,$ X$2,$ X$3,$ X$4)$ # b$1$