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Guess which one is this? Which bone?

analyse_media
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Looking at your sketch: this isn't a single bone but a diagram of the bony pelvis, specifically the pelvic outlet as drawn for obstetric pelvimetry.
What the drawing represents:
  • The large outer oval = the pelvic brim/pelvis outline
  • The two small circles at the bottom = ischial tuberosities (parts of the ischium)
  • The V-shaped lines meeting at a point in front = the pubic rami, forming the pubic arch (part of the pubis)
  • The central oval = the plane of the pelvic outlet (where the fetal head passes)
  • The curved line at the very bottom = the sacrococcygeal junction (sacrum and coccyx)
So the bones involved are the ischium, pubis, sacrum, and coccyx - collectively the posterior and inferior parts of the bony (true) pelvis, viewed from below as it's taught in obstetrics.
The numbers make sense in that context - they're the two ways of measuring the antero-posterior diameter of the outlet:
  • Anatomical outlet AP diameter ≈ 13 cm: measured from the tip of the coccyx to the lower border of the pubic symphysis.
  • Obstetric (functional) outlet AP diameter ≈ 11 cm: measured from the sacrococcygeal joint (not the coccyx tip) to the pubic symphysis, because the coccyx can be pushed backward during delivery, effectively increasing the usable space.
  • The transverse diameter (~10.5-11 cm) is the distance between the ischial tuberosities (bi-ischial/intertuberous diameter).
  • Your "obstructed outlet" column with only 11 cm and 10.5 cm (no anatomical value marked) is showing the reduced/functional measurements used when assessing whether the outlet is adequate for vaginal delivery.
So to directly answer "which bone" - it's really a composite: ischium + pubis (pubic arch) + sacrum/coccyx, i.e., the pelvic outlet of the bony pelvis, not one isolated bone.
If you want, I can pull an actual anatomical diagram of the pelvic outlet with these landmarks labeled for comparison - just say so.
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