**Core Concept**
Non-immunized susceptible contacts of diphtheria are at risk of developing the disease, which can be prevented by administering antibiotics, antitoxin, and the diphtheria toxoid-containing vaccine. This approach helps eliminate the carrier state, reduces the risk of disease transmission, and provides immunity against future infections.

**Why the Correct Answer is Right**
Administering penicillin (C) helps eliminate the carrier state by eradicating the C. diphtheriae bacteria. Diphtheria antitoxin (C) neutralizes the circulating diphtheria toxin, preventing further tissue damage. The DPT vaccine (C) provides immunity against future diphtheria infections by stimulating the production of antibodies against the diphtheria toxin. This combination of treatments is crucial in preventing the spread of diphtheria and reducing the risk of disease in susceptible contacts.

**Why Each Wrong Option is Incorrect**
**Option A:** Erythromycin is an alternative antibiotic for treating diphtheria, but it does not provide immunity against future infections like the DPT vaccine does.

**Option B:** Penicillin alone is insufficient for preventing future infections, as it does not provide immunity. Diphtheria antitoxin is essential, but it does not eliminate the carrier state.

**Option D:** No treatment is required, which is incorrect because non-immunized susceptible contacts are at significant risk of developing diphtheria and transmitting it to others.

**Clinical Pearl / High-Yield Fact**
The DPT vaccine is crucial in preventing diphtheria, as it provides immunity against future infections. It is essential to administer the vaccine to non-immunized susceptible contacts, in addition to antibiotics and antitoxin, to prevent the spread of diphtheria and reduce the risk of disease.

**✓ Correct Answer: C. Penicillin, diphtheria antitoxin and DPT vaccine**