**Core Concept**
Tissue thromboplastin, also known as tissue factor, plays a crucial role in the initiation of the extrinsic coagulation pathway. It is a transmembrane glycoprotein that, upon exposure to blood, binds to and activates factor VII, leading to a cascade of reactions that ultimately result in the formation of a blood clot.

**Why the Correct Answer is Right**
The correct answer is factor VII. When tissue thromboplastin comes into contact with factor VII, it forms a complex that is then converted to activated factor VII (VIIa) by limited proteolysis. This activated factor VIIa then binds to and activates factor IX and factor X, leading to the amplification of the coagulation cascade.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor II (prothrombin) is involved in the common pathway of coagulation, but it is not directly activated by tissue thromboplastin.
**Option B:** Factor VIII is a cofactor in the intrinsic pathway of coagulation and is not directly activated by tissue thromboplastin.
**Option C:** Factor X is a key enzyme in the common pathway of coagulation, but it is not directly activated by tissue thromboplastin.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that tissue thromboplastin is a non-enzymatic protein that initiates the extrinsic coagulation pathway, whereas the intrinsic pathway is initiated by exposure of blood to negatively charged surfaces, such as glass or collagen.

**Correct Answer:** A. Factor VII.