UPSC NDA Math Model Paper 1

Given: Equation of parabola is x=−16y
The given equation of parabola can be re-written as: x2=−4⋅4y ----------(1)
Now by comparing the equation (1) with x2=−4ay we get
⇒a=4
As we know that, equation of directrix of the parabola of the form x2=−4ay is given by: y = a
So, the equation of directrix of the given parabola is: y = 4
As we know that, equation of axis of the parabola of the form x2=−4ay is given by: x = 0
Hence, option D is the correct answer.