Seth is using the figure shown below to prove pythagorean theorem using triangle similarity:

in the given triangle abc, angle a is 90o and segment ad is perpendicular to segment bc.

the figure shows triangle abc with right angle at a and segment ad. point d is on side bc.

which of these could be a step to prove that bc2 = ab2 + ac2?

by the cross product property, ab2 = bc multiplied by bd.
by the cross product property, ac2 = bc multiplied by bd.
by the cross product property, ac2 = bc multiplied by ad.
by the cross product property, ab2 = bc multiplied by ad.