Do same-side interior angles have to be on the same (in this case) horizontal line? to specify, does it always have to be ab, or can it be ad. i’m doing a transversal project at the moment, and am very confused.

if the horizontal lines are all parallel, then yes, as long as the angles are on the same side. If the horizontal lines are parallel but the angles are on opposite sides, then no. If the horizontal lines are not parallel, then it would require more work to figure it out