The construction of altitude ad for triangle def is shown below.

which step in the construction of the altitude assures that ad is perpendicular to ef?

place the compass on vertex d and adjust the width so that the drawn arc goes past side ef.
draw an arc to intersect side ef twice.
from each intersection point, draw an arc above side ef so that the two arcs cross.
draw a line from vertex d to point a to create altitude ad.