**Core Concept**
The success of a free gingival graft procedure is primarily dependent on achieving adequate blood supply to the grafted tissue, which is crucial for its survival and integration with the host site.

**Why the Correct Answer is Right**
The key factor in the success of a free gingival graft is the establishment of a stable blood supply to the grafted tissue. This is achieved by ensuring that the grafted tissue is in close proximity to the underlying bone, allowing for the development of a vascular connection between the graft and the host site. The presence of a sufficient number of blood vessels and a well-organized vascular network within the graft also plays a critical role in its survival and integration.

**Why Each Wrong Option is Incorrect**
**Option A:** Thickness of the graft is a factor in free gingival graft procedures, but it is not the primary determinant of success. While a thicker graft may provide more tissue for coverage, a thinner graft can still be successful if it is properly secured and receives adequate blood supply.

**Option B:** The recipient site's surface area is an important consideration in free gingival graft procedures, as it affects the amount of tissue that can be covered. However, it is not the primary factor in determining the success of the procedure.

**Option C:** The use of a split-thickness graft is not a characteristic of free gingival graft procedures. Free gingival grafts are typically full-thickness grafts, which provide more tissue for coverage and are less likely to contract.

**Clinical Pearl / High-Yield Fact**
To ensure the success of a free gingival graft procedure, it is essential to create a recipient site that is bleeding upon incision, indicating adequate blood supply and a well-vascularized bed for the graft.

**Correct Answer: D.**