**Core Concept**
Proper space between the floor of the mouth and the gingival margins of teeth is crucial for the placement and function of a lingual bar in removable partial dentures. This space ensures adequate clearance for the bar, preventing irritation to the tissues and promoting comfortable wear.

**Why the Correct Answer is Right**
The minimum recommended vertical space for a lingual bar is 10 mm. This space allows for the bar to sit comfortably between the floor of the mouth and the gingival margins, preventing contact with the tissues and reducing the risk of irritation. A space of 10 mm also provides sufficient clearance for the bar to move freely in the mouth, facilitating proper function and stability of the denture.

**Why Each Wrong Option is Incorrect**
**Option A:** A space of 5 mm is too small, leading to potential irritation of the tissues and discomfort for the patient.
**Option B:** A space of 15 mm is excessive, resulting in a loose fit and potential instability of the denture.
**Option C:** A space of 7 mm is insufficient, compromising the clearance and stability of the lingual bar.

**Clinical Pearl / High-Yield Fact**
When designing a lingual bar, it is essential to consider the individual's anatomy and the specific needs of their denture. A thorough examination of the floor of the mouth and the gingival margins is necessary to determine the optimal space for the bar, ensuring a comfortable and functional denture.

**Correct Answer: C. 10 mm**