**Core Concept**
The transmission of stimulus across dentin is a complex process involving the movement of ions and fluids within the dentinal tubules. This process is influenced by the permeability of the dentinal tubules and the presence of various ions, including calcium and hydrogen.

**Why the Correct Answer is Right**
The correct answer involves the movement of ions, particularly calcium and hydrogen, across the dentinal tubules. This process is facilitated by the dentinal fluid, which is a dynamic mixture of ions and water that flows through the dentinal tubules. The movement of these ions is influenced by the concentration gradient and the presence of various ion channels and pumps within the dentinal tubules. The correct answer is related to the movement of calcium ions, which play a crucial role in the transmission of stimulus across dentin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the process of transmission across dentin. While hydrostatic pressure may play a role in the movement of dentinal fluid, it is not the primary mechanism involved in the transmission of stimulus across dentin.

**Option B:** This option is incorrect because it is too vague and does not accurately describe the specific process of transmission across dentin. The movement of ions, particularly calcium and hydrogen, is a critical aspect of this process.

**Option C:** This option is incorrect because it is not directly related to the transmission of stimulus across dentin. While the dentinal tubules are indeed involved in this process, the correct answer is more specific to the movement of ions.

**Clinical Pearl / High-Yield Fact**
The transmission of stimulus across dentin is influenced by various factors, including the presence of fluoride, which can increase the permeability of the dentinal tubules and enhance the movement of ions.

**Correct Answer:** D.