**Core Concept**
MgCl2 (magnesium chloride) is added to the polio vaccine to enhance its thermal stability, allowing for storage and transportation at higher temperatures without compromising its potency.

**Why the Correct Answer is Right**
MgCl2 acts as a stabilizer by inhibiting the growth of certain microorganisms and protecting the vaccine from heat-induced degradation. This is particularly important for the polio vaccine, which must be transported to remote areas in developing countries where refrigeration may not be available. By maintaining the vaccine's potency at higher temperatures, MgCl2 ensures that the vaccine remains effective even in challenging environmental conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** MgCl2 does not potentiate the vaccine; rather, it helps maintain its stability and effectiveness.
**Option C:** MgCl2 is not a preservative; its primary function is to enhance thermal stability, not to prevent microbial growth.

**Clinical Pearl / High-Yield Fact**
When handling vaccines, it's essential to remember that stabilizers like MgCl2 are added to maintain the vaccine's potency and effectiveness, even in adverse environmental conditions.

**✓ Correct Answer: B. Vaccine can be kept at higher temperatures**