The answer is d. The reason this is the answer is because to find the area of a triangle you will find half of the base times the height. The formula looks like this: A=1/2bh.
On this picture the base is the length of the bottom part of the triangle (c). It is the distance of the solid line only.
The height is represented as the straight line that is perpendicular to the base (forms a right/90 degree angle). X is the height of the triangle.
