**Core Concept**
Subunit vaccines are composed of a single component of a pathogen, such as a protein or a polysaccharide, rather than the entire microorganism. This approach is used to stimulate an immune response without the risks associated with live or attenuated vaccines.

**Why the Correct Answer is Right**
Subunit vaccines work by presenting a specific antigen to the immune system, which recognizes and mounts a response against the pathogen. Examples of subunit vaccines include the Hepatitis B vaccine, which contains the HBsAg protein, and the Hib vaccine, which contains the polysaccharide capsule of Haemophilus influenzae type b. These vaccines are often used in individuals who cannot receive live or attenuated vaccines, such as those with weakened immune systems.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but subunit vaccines do not typically include whole microorganisms, so options that include live or attenuated organisms would be incorrect.
* **Option B:** This option is not provided, but subunit vaccines can include proteins, but not the entire cell wall or peptidoglycan layer, so options that include these components would be incorrect.
* **Option C:** This option is not provided, but subunit vaccines can include polysaccharides, but not the entire capsule or surface antigens, so options that include these components would be incorrect.
* **Option D:** This option is not provided, but subunit vaccines can include recombinant proteins, but not the entire virus or bacteria, so options that include these components would be incorrect.

**Clinical Pearl / High-Yield Fact**
Subunit vaccines are a key component of vaccine development, as they offer a safe and effective way to stimulate an immune response without the risks associated with live or attenuated vaccines. Examples of subunit vaccines include the HPV vaccine, which contains recombinant proteins from the L1 and L2 capsid proteins of human papillomavirus.

**Correct Answer:**
(Unfortunately, the correct answer options are not provided, but based on the options listed above, the correct answer would likely be one of the options that includes a protein or polysaccharide component.)