**Core Concept**
The minimum bone width required for a dental implant is a critical factor in determining the success of the implant. It is essential to ensure that the bone width is adequate to accommodate the implant without compromising its stability or the surrounding bone.

**Why the Correct Answer is Right**
The minimum bone width required for a 4 mm diameter implant is generally considered to be 6 mm. This is because the implant needs to be surrounded by a sufficient amount of bone to provide primary stability and prevent bone loss over time. A bone width of 6 mm ensures that there is enough bone to cover the implant, prevent bone fenestration, and allow for osseointegration. The implant's diameter is also a factor, as a 4 mm diameter implant requires a slightly larger bone width than a smaller diameter implant.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a sufficient bone width to ensure primary stability and prevent bone loss.
* **Option B:** This option is incorrect because it is too narrow, which can lead to implant failure due to lack of bone coverage.
* **Option C:** This option is incorrect because it is not a standard bone width for a 4 mm diameter implant, and may not provide adequate stability or bone coverage.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the bone width, implant diameter, and type of implant when planning dental implant placement. A thorough evaluation of the patient's bone morphology and density is crucial to ensure the success of the implant.

**Correct Answer: D. 6 mm.**