**Core Concept**
The obstetric conjugate is the shortest distance between the promontory of the sacrum and the pubic symphysis, which is a crucial measurement in obstetrics to assess the pelvic inlet. The **conjugate** is an important factor in determining the possibility of a normal vaginal delivery.

**Why the Correct Answer is Right**
The correct answer is based on the relationship between the conjugate of the diagonal and the obstetric conjugate. When the conjugate of the diagonal is taken as 'a' cm, the obstetric conjugate can be estimated using a formula that subtracts a fixed value from the conjugate of the diagonal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for the subtraction of a fixed value.
**Option B:** This option does not apply the correct formula to estimate the obstetric conjugate.
**Option C:** This option also does not apply the correct formula.

**Clinical Pearl / High-Yield Fact**
The obstetric conjugate is typically around 10 cm or more in most women, which allows for a normal vaginal delivery.

**Correct Answer:** Correct Answer: D. a - 2