b
$\text { Here, } \vec{F}_{B C}=\vec{F}$
$\because \quad \vec{F}_{A B}=0$
The net magnetic force on a current carrying closed loop in a uniform magnetic field is zero.
$\therefore \vec{F}_{A B}+\vec{F}_{B C}+\vec{F}_{A C}=0$
$\Rightarrow \vec{F}_{A C}=-\vec{F}_{B C}$ $\left(\because \vec{F}_{A B}=0\right)$