Explanation

(a)
Let the equation of parabola having the directrix parallel to $x$-axis is
$x^{2} = 4a(y + k)$ …..(i)
and equation of directrix is $y = p$ ….(ii)
Here, 3 unknowns are in Eqs. (i) and (ii).
$\therefore$ Order of DE such parabolas having directrix parallel to $x$-axis is 3.