**Core Concept**
The obstetrical conjugate is an important measurement in obstetrics, referring to the distance between the promontory of the sacrum and the pubic symphysis, which is critical for determining the fetal head's descent and possible delivery route. This measurement is essential for identifying potential complications during childbirth.

**Why the Correct Answer is Right**
The diagonal conjugate, denoted as 'a' cm, is a direct measurement taken from the pubic bone to the sacral promontory. The obstetrical conjugate is approximately 1.5 to 2 cm less than the diagonal conjugate. This is because the obstetrical conjugate is measured along a more curved line, which is the path the fetal head takes during delivery. The difference in measurements is due to the curvature of the pelvic inlet, which affects the fetal head's descent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it suggests a direct correlation between the diagonal conjugate and the obstetrical conjugate, ignoring the curvature of the pelvic inlet.
**Option B:** This option is incorrect as it suggests a fixed ratio between the two conjugates, which does not account for individual variations in pelvic anatomy.
**Option C:** This option is incorrect as it implies a much larger difference between the two conjugates than is actually the case.

**Clinical Pearl / High-Yield Fact**
In obstetrics, a conjugate measurement of less than 10 cm is considered a significant factor in the decision to perform a cesarean delivery, as it may indicate a difficult vaginal delivery.

**Correct Answer: C. 10-12 cm.**