Shortest diameter is:

Ans. is b, i.e. Obstetric conjugateRef. Dutta Obs. 9/e, p 80-81Anteroposterior diameters of the pelvic inlet.DiametersFeatureMeasurement IObstetric conjugate* It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis.* It is the smallest AP diameter of pelvic inlet.* It is the diameter through which the fetus must pass.* It can not be measured clinically, but can be derived by subtracting 1.5 cm from diagonal conjugate.10-10.5 cmTrue conjugate(Anatomical conjugate)* It is the distance between the midpoint of the sacral promontory to the inner margin of the upper border of symphysis pubis.* It has no obstetrical significance.11 cmDiagonal conjugate* It is the distance between the midpoint of the sacral promontory to the lower border of symphysis pubis.* Its importance as that it can be measured clinically.12 cm