Write the equation of a parabola with its vertex at the origin and directrix at y = 1.

A parabola with vertex at the origin and a horizontal directrix has equation x² = 4py, where its directrix is y = −p. Since the directrix is y = 1, p = −1, so the equation is x² = −4y. Therefore, the correct choice is C, x² = −4y.

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