**Core Concept**
The adequacy of the pelvis for vaginal delivery is determined by various clinical and radiological findings that assess the dimensions and shape of the pelvic outlet, mid-pelvis, and pelvic inlet. These findings are crucial in predicting the likelihood of a successful vaginal delivery.

**Why the Correct Answer is Right**
Adequate pelvic dimensions for vaginal delivery include a pelvic inlet with a transverse diameter of at least 13 cm, a mid-pelvis with a transverse diameter of at least 12 cm, and a pelvic outlet with a transverse diameter of at least 12 cm. The pelvis is also assessed for its shape, with a more oval shape being more favorable for vaginal delivery. The presence of a well-formed pelvic cavity and adequate pelvic floor muscles further support the adequacy of the pelvis for vaginal delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may refer to the presence of a well-formed pelvic cavity, but the absence of this feature is not a specific contraindication for vaginal delivery. In fact, a well-formed pelvic cavity is a desirable feature for vaginal delivery.

**Option B:** This option is incorrect as a narrow pelvic inlet is not a contraindication for vaginal delivery. The pelvic inlet is assessed for its transverse diameter, and a diameter of at least 13 cm is generally considered adequate for vaginal delivery.

**Option C:** This option is incorrect as a well-formed pelvic floor is a desirable feature for vaginal delivery. The presence of a well-formed pelvic floor muscles supports the adequacy of the pelvis for vaginal delivery.

**Option D:** This option is incorrect as a well-formed pelvic cavity is a desirable feature for vaginal delivery. The presence of a well-formed pelvic cavity supports the adequacy of the pelvis for vaginal delivery.

**Clinical Pearl / High-Yield Fact**
The obstetric conjugate, which is the distance from the lower margin of the pubic symphysis to the sacral promontory, is a useful measurement in assessing the adequacy of the pelvis for vaginal delivery. A measurement of at least 10 cm is generally considered adequate for vaginal delivery.

**Correct Answer:** D.