To replace the force couple system at $O$ by a single force, the line of action of the force must pass through $-----$

$0\mathrm{.}4m$ on the $x-$axis

$0\mathrm{.}4m$ on the $y-$axis

$--0\mathrm{.}4m$ on the $x-$axis

$--0\mathrm{.}4$ m on the $y-$axis