**Core Concept**
The question is testing the mechanism of action of streptokinase, a thrombolytic agent used to dissolve blood clots. Streptokinase works by activating the fibrinolytic system, which breaks down fibrin clots. This process involves the binding of streptokinase to a specific protein on the surface of endothelial cells.

**Why the Correct Answer is Right**
Streptokinase binds to plasminogen, a circulating protein that is converted to plasmin, a fibrinolytic enzyme. Plasmin then breaks down fibrin clots into smaller fibrin degradation products. This process is crucial for restoring blood flow in cases of acute myocardial infarction. The binding of streptokinase to plasminogen is a specific, high-affinity interaction that triggers the subsequent steps in the fibrinolytic cascade.

**Why Each Wrong Option is Incorrect**
**Option A:** Plasmin itself is the enzyme that breaks down fibrin clots, but it is not the protein to which streptokinase binds.
**Option B:** Fibrinogen is a coagulation factor involved in clot formation, but it is not the target of streptokinase.
**Option C:** Tissue plasminogen activator (tPA) is another thrombolytic agent that works by activating plasminogen, but it is not the protein to which streptokinase binds.

**Clinical Pearl / High-Yield Fact**
Streptokinase is a foreign protein that can cause an immune response, which may lead to anaphylaxis in some patients. This is why tPA is often preferred as a thrombolytic agent in acute myocardial infarction.

**Correct Answer:** B. Plasminogen.