The directrix of the parabola x^2+6x-8y+25=0 is the line

Complete the square: $(x+3)^2=8y-16=8(y-2)$. This is $X^2=4aY$ with $4a=8$, so $a=2$, vertex $(-3,2)$, and the parabola opens upwards. The directrix lies $a$ below the vertex, so it is $y=2-2=0$. The line $y=4$ is the latus-rectum line through the focus $(-3,4)$, and the vertical lines $x=-1$ and $x=-5$ are parallel to the axis, which cannot be directrices of this upward parabola.