Which step in the construction of the altitude assures that AD is perpendicular to EF?

The two intersection points on EF are the same distance from D. Drawing equal-radius arcs from those points so they cross above EF creates a point A that is also equally distant from both intersections. The line joining D to A is therefore the perpendicular bisector of the segment between them, so AD is perpendicular to EF. Choice C is the step that ensures this perpendicular relationship.

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