**Core Concept**
Oral Poliovirus Vaccine (OPV) requires stringent storage conditions due to its live, attenuated nature. The vaccine contains weakened poliovirus strains that can revert to virulence if not stored properly. This necessitates strict adherence to cold chain maintenance to prevent degradation and potential vaccine-derived poliovirus (VDPV) emergence.

**Why the Correct Answer is Right**
OPV is a live, attenuated vaccine, meaning it contains weakened forms of the poliovirus. To prevent the vaccine virus from reverting to its virulent form, OPV must be stored at a temperature range of -20°C to -15°C. If the vaccine is not stored within this narrow temperature range, the live virus can multiply uncontrollably, leading to a loss of potency or even the emergence of VDPVs. This stringent storage requirement makes OPV more challenging to handle and transport than other vaccines.

**Why Each Wrong Option is Incorrect**
**Option A:** DPT (Diphtheria, Pertussis, and Tetanus) vaccine is a killed-virus vaccine, which does not require the same level of stringent storage conditions as live, attenuated vaccines like OPV.

**Option C:** BCG (Bacillus Calmette-Guérin) vaccine is a live, attenuated vaccine, but it can be stored at refrigerator temperatures (2°C to 8°C), making it less demanding than OPV in terms of storage requirements.

**Option D:** TT (Tetanus Toxoid) vaccine is a toxoid-based vaccine, which is inactivated and does not require the same level of stringent storage conditions as live, attenuated vaccines like OPV.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) recommends that all vaccines, especially live, attenuated vaccines like OPV, be stored at -20°C to -15°C to maintain their potency and prevent the emergence of VDPVs.

**Correct Answer:** ✓ Correct Answer: B. OPV