Answer
$x^2=y$
Work Step by Step
We are given that the directrix is a horizontal line. This implies that the parabola is vertical.
The standard form of a vertical parabola is given as: $x^2=4py$ when vertex is centered at origin.
Here, the directrix is $y=-p \implies -p=-\dfrac{1}{4} \implies p=\dfrac{1}{4}$
Now, we have:
$$x^2=(4)\left(\dfrac{1}{4}\right) y \\
\implies x^2=y$$
