**Core Concept**
The complete cleft of the secondary palate involves the total separation of the hard and soft palate, resulting in a significant defect in the midfacial structure. This condition arises from the failure of the palatine shelves to fuse during embryonic development.

**Why the Correct Answer is Right**
In a complete cleft, the hard palate is separated from the alveolar ridge and the maxillary bone. This occurs due to the incomplete closure of the palatine shelves, which are formed by the palatine shelves' failure to meet and fuse with the medial nasal prominences. The resulting gap in the hard palate can significantly affect the patient's ability to eat, speak, and maintain oral hygiene.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the hard palate is not separated from the soft palate in a complete cleft, but rather from the alveolar ridge and maxillary bone.
**Option B:** This option is incorrect because the hard palate is not separated from the vomer bone in a complete cleft, but rather from the alveolar ridge and maxillary bone.
**Option C:** This option is incorrect because the hard palate is not separated from the nasal septum in a complete cleft, but rather from the alveolar ridge and maxillary bone.

**Clinical Pearl / High-Yield Fact**
In complete clefts, the hard palate is often separated from the alveolar ridge and maxillary bone, which can lead to difficulties in speech and feeding, as well as an increased risk of dental and orthodontic problems.

**Correct Answer:** D.