**Core Concept**
Vitamin K plays a crucial role in the post-translational modification of certain proteins, specifically those involved in blood coagulation. It serves as a cofactor for the enzyme gamma-glutamyl carboxylase, which is essential for the activation of several coagulation factors.

**Why the Correct Answer is Right**
Vitamin K-dependent proteins, such as prothrombin (factor II), factors VII, IX, and X, undergo a process called gamma-carboxylation. This process involves the addition of a carboxyl group to specific glutamic acid residues, allowing these proteins to bind calcium ions and interact with phospholipid surfaces. This activation is critical for the proper functioning of the coagulation cascade. The enzyme gamma-glutamyl carboxylase is responsible for this modification, and vitamin K serves as a cofactor for this enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamin K is not directly involved in the activation of platelets.

**Option B:** While vitamin K is essential for blood coagulation, it is not directly involved in the activation of fibrinogen.

**Option C:** This option is incorrect because vitamin K is not directly involved in the activation of plasminogen.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that vitamin K deficiency can lead to bleeding disorders, such as hemophilia or easy bruising. Additionally, warfarin, an anticoagulant, works by inhibiting vitamin K-dependent gamma-carboxylation, which can lead to an increased risk of bleeding.

**Correct Answer:** D.