Answer:
D. y = 0
Step-by-step explanation:
If a parabola has the following form:

The directrix of the parabola will be the line with an equation:
y = k - p
In this case, we have:

So, comparing the equations, we can say that:
4p = 4
k = 1
It means that the value of p can be calculated as:
4p = 4
4p/4 = 4/4
p = 1
Therefore, the equation of the directrix is:
y = k - p
y = 1 - 1
y = 0
So, the answer is D. y = 0