Answer
$\phi = 0^{\circ}~~$ or $~~\phi = 180^{\circ}$
Work Step by Step
We can find the angle $\phi$ between the directions of $r$ and $F$:
$\tau = r ~F~sin~\phi = 0$
$(3~m)(4~N)~sin~\phi = 0$
$sin~\phi = 0$
$\phi = 0^{\circ}~~$ or $~~\phi = 180^{\circ}$
Note that $r$ and $F$ point in the same direction or in completely opposite directions.
