Tooth reduction for anterior tooth preparation for porcelain fused to metal and all ceramic crowns is dictated by the following imperative(s)
1. length for adequate retention-resistance.
2. porcelain/ceramic thickness for fracture resistance.
3. clearance for occlusal function.
4. parallelism of axial walls for facilitating the path of insertion.

All of the above.

(1) (2) (3)

(1) and (3)

(2) and (4)

(4) only