If the directrix is horizontal, then the parabola opens (text{horizontally}/text{vertically})

Step 1
Given:
We must find the missing word in the given sentence.
So, Definition of parabola said:
A parabola is another type of conic section generated by the cross section of a cone intersected by a plane.
An equation of the form:
${\displaystyle y=a{x}^2+bx+c(a\ne 0)}$
Step 2
The above equation is parabola opening upward if ${\displaystyle a>0}$ and opening downward if ${\displaystyle a<0.}$
So, the geometrically definition of parabola is the set of all points in a plane that the equidistant from a fixed line called directrix and a fixed point called the focus.
Step 3
In the standard form of a parabola, if x and y are replaced by ${\displaystyle x-h\text{and}y-k}$, then the graph of the parabola is shifted h units horizontally and k units vertically. The vertex is (h, k), and the focus is ${\displaystyle (h,k+p).}$
The directrix is the line defined by the equation ${\displaystyle y=k-p}$ and the equation of the parabola is:
${\displaystyle {(x-h)}^2=4p(y-k)}$
Step 4
The equation of directrix ${\displaystyle y=k-p}$ is horizontal. So, the parabola opens towards vertically.
Thus, the missing word is Vertically.