**Core Concept**
The chemical method of strengthening porcelain involves an ion exchange process to increase its mechanical properties. This is a crucial step in dental ceramics fabrication, as it enhances the durability and resistance of the material to wear and fracture.

**Why the Correct Answer is Right**
The correct answer involves the exchange of sodium and potassium ions. During the chemical method, the porcelain is immersed in a molten salt bath, typically containing a mixture of sodium nitrite and potassium nitrite. The sodium ions (Na+) are exchanged with the potassium ions (K+) from the bath, resulting in the formation of sodium nitrite (NaNO2) and potassium ions that are incorporated into the porcelain. This ion exchange process leads to the strengthening of the porcelain by reducing its porosity and increasing its density.

**Why Each Wrong Option is Incorrect**
**Option A:** Exchange of aluminium and sodium ions is incorrect because the aluminium ions are not directly involved in the ion exchange process during the chemical method of strengthening porcelain.

**Option B:** Exchange of aluminium and potassium ions is incorrect because, as mentioned earlier, aluminium ions are not directly involved in this process.

**Option D:** None of the above is incorrect because the correct answer is indeed an exchange of sodium and potassium ions.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the chemical method of strengthening porcelain is a critical step in dental ceramics fabrication, and the ion exchange process can significantly affect the material's mechanical properties.

**✓ Correct Answer: C. exchange of sodium and potassium ions**