The directrix for a parabola with a standard form of (y − k)2 = 4p(x − h) is

In (y − k)² = 4p(x − h), the parabola opens horizontally, with vertex (h, k) and focus (h + p, k). Its directrix is the vertical line p units in the opposite direction from the vertex, so its equation is x = h − p. Therefore, the correct choice is D, x = h − p.

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