What is the area of the triangle with vertices at (1, 1), (5, 1), and (3, 7)?

✓ Correct Answer: Option A

The base of the triangle lies on y=1, from x=1 to x=5, so its length is 5-1=4. The height is the perpendicular distance from (3,7) to y=1, which is 7-1=6. Area = (1/2) * base * height = (1/2) * 4 * 6 = 12 square units.