Use the vertical side as the base. The base has length y2 - y1.
Use a segment drawn from vertex (x3, y3) perpendicular to the base as the height. The height has length x3 - x1.
area = base * height/2
area = (1/2)(y2 - y1)(x3 - x1)
Answer: D. (1/2)(y2 - y1)(x3 - x1)