**Core Concept**
Hemophilia B, also known as Christmas disease, is a genetic disorder caused by a deficiency of a specific clotting factor in the blood. This deficiency leads to impaired blood coagulation and increased bleeding tendency.

**Why the Correct Answer is Right**
Hemophilia B is caused by a deficiency of factor IX, a vitamin K-dependent clotting factor that plays a crucial role in the intrinsic pathway of blood coagulation. Factor IX is essential for the conversion of factor X to its activated form, which then triggers the coagulation cascade and leads to the formation of a blood clot. A deficiency of factor IX results in prolonged bleeding after minor injuries and spontaneous bleeding episodes.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor VIII is the correct answer for Hemophilia A, not Hemophilia B. Factor VIII is another vitamin K-dependent clotting factor involved in the intrinsic pathway of blood coagulation.
**Option B:** Factor II is involved in the extrinsic pathway of blood coagulation and is not directly related to Hemophilia B.
**Option C:** Factor VII is also involved in the extrinsic pathway of blood coagulation and is not related to Hemophilia B.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Hemophilia B is inherited in an X-linked recessive pattern, meaning that the gene responsible for the disorder is located on the X chromosome. Males are more commonly affected than females because they have only one X chromosome.

**Correct Answer:** D. Factor IX.