3) with question, marking brainliest + !

A directrix can be viewed as a "vector" for the points of the focus of a quadratic function. In other words, like a hypotenuse, it can be calculated using Pythagorean Theorem.
Since d: x = 8, set that equal to 0
d: x-8 = 0
Same thing for the focal points:
x: x = -8 --> x+8 = 0
y: y = 0 --> y -0 = 0 --> y = 0
Now use the value of d equal to zero as the "vector" of the x and y values of the focus. In mathematical terms:

where c = (x-8), a = (x+8), and b = (y)

Now get the x's and y on opposite sides:

therefore, D) is the correct one