A parabola has a vertex at the origin. The equation of the directrix of the parabola is y = 3.

The directrix y = 3 is a horizontal line 3 units above the vertex at (0, 0). The focus lies the same distance from the vertex on the opposite side, so it is 3 units below the origin at (0, −3). Therefore, the correct choice is C, (0, −3).

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