The correct answer is: A. Straight line path.
In a graded index fiber, the refractive index of the core decreases gradually from the center to the cladding. This causes the light to be refracted multiple times as it travels down the fiber, but the total path of the light is a straight line.
A parabolic path is the path of a projectile that is launched at an angle to the ground. The projectile will travel in a curved path that is shaped like a parabola.
An elliptical path is the path of a planet or other celestial body that orbits around a star. The path of the planet is shaped like an ellipse, with the star at one focus of the ellipse.
A circular path is the path of an object that is moving in a circle. The object will travel in a constant radius around the center of the circle.
In a graded index fiber, the light is not traveling in a parabolic, elliptical, or circular path. The light is traveling in a straight line.