'Torque' in orthodontics refers to:
Clinical heights of the crowns, contact areas, their tip (mesio distal angulations) and torque (labiolingual inclination) contribute to the aesthetics of the smile. 
Therefore torque refer to the labiolingual inclination of the teeth.
Reference: Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities 3rd O P Kharbanda, pdf no 789
The change in mesiodistal inclination of teeth
The change in labiolingual inclination of teeth
The rotation of teeth
None of the above