Which of the following is not a primary resistance form?
Primary resistance form is obtained through use of a preparation design that conserves as much healthy tooth structure as possible. 
Transitions between the walls of the preparation (i.e., the internal line angles) are slightly rounded so as to limit stress concentration in these areas, which increases tooth resistance to fracture. Rounding of external angles within the tooth preparation (e.g., axiopulpal line angles) limits the likelihood of stress concentration in the corresponding intaglio surface of restorative materials, which increases resistance to fracture of the restorative material. The relatively horizontal pulpal and gingival walls, prepared perpendicular to the tooth’s long axis, help the restoration resist occlusal forces and limit the likelihood of tooth fracture from wedging effects caused by opposing cusps.
Note : Occlusal convergence is a retention form.
Reference: Sturdevant operative dentistry 7th EDITION, Page No:127
Rounding of internal line angle
Rounding of axiopulpal line angle
Occlusal convergence
Horizontal pulpal and gingival walls