**Core Concept**
The question requires an understanding of the morphology and classification of teeth in the human dentition. The correct answer will be based on the relative size of posterior teeth, which are typically classified as molars and premolars.

**Why the Correct Answer is Right**
The human dentition includes four types of posterior teeth: first and second premolars (also known as bicuspids), first and second molars, and third molars (also known as wisdom teeth). Among these, the second premolar is generally the smallest posterior tooth due to its smaller size and simpler morphology compared to the larger molars. The second premolar has two cusps and a smaller overall size, distinguishing it from the larger molars with three or more cusps.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because, although the first premolar is smaller than the second molar, it is still larger than the second premolar.
* **Option B:** This option is incorrect because the second molar is one of the larger posterior teeth, making it the smallest option.
* **Option D:** This option is incorrect because the third molar (wisdom tooth) is typically larger and more variable in size compared to the second premolar.

**Clinical Pearl / High-Yield Fact**
When examining the posterior teeth, remember that the second premolar is often the smallest tooth in the posterior region, making it an important landmark for dental restorations and oral surgery procedures.

**Correct Answer:** C. Second premolar.