Note that the given line affects the angle A. A line is an angle bisector if it splits an angle into two angles that have the same measure. Visually, we can see that the two angles formed by the line AD have different measures. So, option C is discarded.
Also, note that segment BD has a different length from segment DC. Since this segments have different lengths, it means that line ADdoes not bisect the line BC as if it did, both segments should have the same length.
Note that vertex A is opposite to the side BC. Also, note that the line AD connects the vertex A to its opposite side forming angles of 90°. So, it this means that line AD is perpendicular to line BC.
Recall that the altitude of a triangle is a line segment from a vertex that is perpendicular to the opposite side. So, based on the previous description, the line AD is an altitude of the triangle. Hence, the correct is option D.