Which of the following is the general format of an objective function?

a$.$ f$($X$1,$ X$2,..,$ Xn$)$

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

c$.$ f$($X$1,$ X$2,..,$ Xn$)<=$ b

d$.$ f$($X$1,$ X$2,...,$ Xn$)=$ b