The correct answer is: D. Straight line
The locus of the end point of the resultant of the normal and tangential components of the stress on an inclined plane is a straight line. This is because the normal and tangential components of the stress are always perpendicular to each other, and the resultant of two perpendicular vectors is a straight line.
The normal component of the stress is the component of the stress that is perpendicular to the surface of the inclined plane. The tangential component of the stress is the component of the stress that is parallel to the surface of the inclined plane.
The resultant of the normal and tangential components of the stress is the vector sum of these two components. The resultant of two perpendicular vectors is a straight line.
Therefore, the locus of the end point of the resultant of the normal and tangential components of the stress on an inclined plane is a straight line.
The other options are incorrect because they are not straight lines. A circle is a closed curve with all points at the same distance from a center point. A parabola is a curve that is symmetrical about a vertical axis and has a point of maximum or minimum value. An ellipse is a closed curve with two axes of symmetry, one major and one minor axis.