**Core Concept**
Classical hemophilia, also known as hemophilia A, is a genetic disorder caused by a deficiency in a specific clotting factor. It leads to prolonged bleeding due to the inability of the blood to form clots properly.

**Why the Correct Answer is Right**
The correct answer is related to the absence of a crucial protein involved in the coagulation cascade. Hemophilia A is caused by a deficiency in factor VIII, a protein that plays a key role in the intrinsic pathway of blood coagulation. In the absence of factor VIII, the blood cannot form a stable fibrin clot, leading to prolonged bleeding. The factor VIII protein interacts with tissue factor and other clotting factors to facilitate the conversion of fibrinogen to fibrin, which forms the clot.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor V deficiency is associated with a different condition, known as factor V deficiency or parahemophilia. This condition is distinct from hemophilia A and has a different clinical presentation.
**Option B:** Factor XII is involved in the intrinsic pathway of coagulation but is not responsible for classical hemophilia. Deficiency in factor XII can lead to an increased risk of bleeding, but it is not the primary cause of classical hemophilia.
**Option C:** Factor XI deficiency is another condition that affects the intrinsic pathway of coagulation. However, it is not the primary cause of classical hemophilia and has a different clinical presentation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that classical hemophilia is caused by a deficiency in factor VIII, which is inherited in an X-linked recessive pattern. This means that the condition is more common in males, who have only one X chromosome, and is often inherited from an affected mother who is a carrier of the disease.

**Correct Answer: D. Factor VIII**