**Core Concept**
A hapten is a small molecule that, on its own, is unable to elicit an immune response but can bind to a larger carrier protein, making it immunogenic. This concept is crucial in understanding allergic reactions and the development of vaccines.

**Why the Correct Answer is Right**
A hapten's inability to stimulate an immune response is due to its small size, which prevents it from being recognized by the immune system on its own. However, when it binds to a carrier protein, the hapten-carrier complex becomes immunogenic, allowing the immune system to recognize and respond to it. This process is essential in the development of vaccines, where small antigenic determinants are often linked to carrier proteins to enhance immunogenicity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hapten-carrier complexes are immunogenic, not the haptens themselves.
**Option B:** This option is incorrect because haptens do not stimulate an immune response on their own, but rather require a carrier protein to become immunogenic.
**Option C:** This option is incorrect because haptens are not inherently toxic or pathogenic; their immunogenic potential is dependent on their ability to bind to a carrier protein.

**Clinical Pearl / High-Yield Fact**
When developing vaccines, scientists often use haptens to create immunogenic complexes that can elicit a specific immune response. This concept is essential in understanding how vaccines work and how they can be designed to target specific pathogens.

**Correct Answer: D. Haptens are small molecules that require a carrier protein to become immunogenic.**