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

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

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

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

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