**Core Concept:**
Killed vaccines are forms of vaccines that use inactivated pathogens. They contain pathogens that have been killed or chemically inactivated, preventing them from causing disease but preserving their immunogenicity. This leads to the generation of an immune response without the risk of the actual disease. Live attenuated vaccines and whole-cell vaccines are examples of killed vaccines.

**Why the Correct Answer is Right:**
Killed vaccines work by stimulating the immune system to recognize the pathogens as foreign, leading to the production of antibodies and memory cells. The immune system then remembers these pathogens, making it quicker to mount an effective immune response if the actual pathogen is encountered in the future.

**Why Each Wrong Option is Incorrect:**
A. Option A is incorrect because live attenuated vaccines contain live, but weakened forms of pathogens, which can cause the disease but with milder symptoms.
B. Option B is incorrect because subunit vaccines use specific parts of the pathogens, like proteins or polysaccharides, rather than the whole pathogen, which is the case in killed vaccines.
C. Option C is incorrect because whole-cell vaccines involve using the whole pathogen, which is also an example of a killed vaccine.
D. Option D is incorrect because conjugate vaccines use a carrier protein to enhance the immune response to the pathogen, which is not a killed vaccine.

**Why Option C is Right:**
Option C (Whole-cell vaccines) is a correct answer because whole-cell vaccines utilize whole pathogens that have been killed or chemically inactivated, stimulating an immune response similar to killed vaccines.

**Clinical Pearl:**
A notable example of a killed vaccine is the Haemophilus influenzae type b (Hib) vaccine, which uses killed bacteria to protect against H. influenzae type b infections.

**Correct Answer:**
Correct Answer: Whole-cell vaccines (Option C)