**Core Concept**
Heparin is a potent anticoagulant that acts by enhancing the activity of antithrombin III, leading to the inhibition of thrombin and other coagulation factors. The antidote for heparin is a medication that counteracts its anticoagulant effects, allowing for the restoration of normal coagulation pathways.

**Why the Correct Answer is Right**
Protamine sulfate is a positively charged polypeptide that binds to the negatively charged heparin molecule, forming a stable complex that inactivates heparin's anticoagulant activity. This binding reaction is highly specific, with protamine sulfate having a 10-fold greater affinity for heparin than for other polyanions. The protamine-heparin complex is then rapidly cleared from the circulation, restoring normal coagulation function.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because protamine sulfate is the primary antidote for heparin, whereas protamine is used to treat bleeding in patients on heparin, but not as the antidote itself.
**Option B:**
This option is incorrect because warfarin is an oral anticoagulant that acts by inhibiting vitamin K-dependent clotting factors, and its antidote is vitamin K, not protamine sulfate.
**Option C:**
This option is incorrect because activated charcoal is used to treat overdose of various medications, but it is not an antidote for heparin.
**Option D:**
This option is incorrect because fresh frozen plasma is used to treat bleeding in patients with clotting factor deficiencies, but it is not an antidote for heparin.

**Clinical Pearl / High-Yield Fact**
To reverse heparin overdose, it is essential to administer protamine sulfate in a dose of 1 mg of protamine for every 100 units of heparin administered in the past 4 hours, or for every 5 units/kg of heparin administered in the past 2 hours.

**Correct Answer: D. Fresh frozen plasma**