**Core Concept**
Oral polio vaccine (OPV) is a live, attenuated vaccine used to prevent poliomyelitis. It contains a weakened form of the poliovirus, which replicates in the gut and induces immunity.

**Why the Correct Answer is Right**
OPV is administered orally, and the weakened virus replicates in the gut, where poliovirus naturally multiplies. This replication leads to a local immune response, which provides immunity against poliovirus. The gut-associated lymphoid tissue (GALT) plays a crucial role in this process, as it is responsible for the initial immune response to the vaccine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because OPV is not a killed-virus vaccine; it contains a live, attenuated virus.
**Option B:** This option is incorrect because OPV is not administered via injection; it is given orally.
**Option C:** This option is incorrect because OPV is not a subunit vaccine; it contains a whole, attenuated virus.

**Clinical Pearl / High-Yield Fact**
OPV is a crucial tool in global polio eradication efforts, but it can cause vaccine-associated paralytic poliomyelitis (VAPP) in rare cases, particularly in areas with poor sanitation and hygiene.

**Correct Answer: D.