**Core Concept**
Post-translational modification of clotting factors refers to the series of biochemical reactions that occur after the initial translation of clotting factor messenger RNA into a protein, which ultimately leads to the activation and maturation of these proteins. This process involves various enzymes and cofactors that modify the clotting factors to achieve their functional forms.

**Why the Correct Answer is Right**
The correct answer involves the enzyme **Von Willebrand Factor (VWF) cleaving protease, also known as ADAMTS13**. This enzyme specifically cleaves and inactivates VWF, which is essential for the regulation of VWF's activity in blood clotting. ADAMTS13 deficiency can lead to thrombotic thrombocytopenic purpura (TTP), a condition characterized by excessive clotting and thrombocytopenia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to post-translational modification of clotting factors. **Factor Xa** is a clotting factor involved in the coagulation cascade, but it is not primarily involved in post-translational modification.
**Option B:** This option is incorrect because **Factor VIII** is a clotting factor that requires post-translational modification, but it does not directly require ADAMTS13 for its activation.
**Option C:** This option is incorrect because **Thrombin** is a clotting enzyme that activates various clotting factors, but it is not primarily involved in post-translational modification of clotting factors.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that ADAMTS13 deficiency can lead to thrombotic thrombocytopenic purpura (TTP), a condition characterized by excessive clotting and thrombocytopenia. This highlights the critical role of ADAMTS13 in regulating VWF activity and preventing excessive clotting.

**Correct Answer:** D. ADAMTS13