Horizontal abduction involves the movement of the arm or thigh in the transverse plane from a(n) position to a lateral position.\

There is a horizontal abduction and horizontal adduction in the transverse plane that is imaginary bisector that divides the body into two halves upper or top and lower or bottom halves.
Horizontal abduction involves the movement of the arm or thigh from a medial or anterior position to a lateral position and in horizontal adduction, the movement is reverse that is from lateral position to anterior position.

Answer ( slope);