**Core Concept**
Vitamin K plays a crucial role in the post-translational modification of clotting factors II, VII, IX, and X, which are synthesized in the liver. These factors require gamma-carboxylation to bind calcium ions and phospholipids, thus enabling them to participate in the coagulation cascade. A deficiency in vitamin K impairs this process, leading to a decrease in the activity of these clotting factors.

**Why the Correct Answer is Right**
In vitamin K deficiency, the gamma-carboxylation of clotting factors II, VII, IX, and X is impaired, resulting in a decrease in their activity. This decrease in clotting factor activity leads to an increase in the levels of their precursors, which are produced in the liver in an attempt to compensate for the deficiency. The most notable increase in vitamin K deficiency is the level of prothrombin (factor II) and its precursors. This increase is due to the impaired gamma-carboxylation of prothrombin, leading to a buildup of its precursors in the liver.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to vitamin K deficiency, as it does not directly relate to the coagulation cascade or the effects of vitamin K on clotting factors.
* **Option B:** This option is incorrect because the decrease in clotting factor activity in vitamin K deficiency leads to a decrease in the levels of these factors, not an increase.
* **Option C:** This option is incorrect because the increase in clotting factor precursors in vitamin K deficiency is not typically associated with an increase in this specific factor.

**Clinical Pearl / High-Yield Fact**
A classic clinical manifestation of vitamin K deficiency is easy bruising or bleeding due to impaired coagulation. This is often seen in patients with malabsorption syndromes, such as celiac disease or Crohn's disease, or in those taking anticoagulant medications.

**Correct Answer: C. Prothrombin.**