**Core Concept**
Platelet function is crucial for primary hemostasis, and assessing its integrity is essential in various clinical scenarios, such as bleeding disorders or thrombotic events. The best index of platelet function would ideally reflect the platelet's ability to aggregate, adhere, and release granules.

**Why the Correct Answer is Right**
The correct answer is **B. Bleeding Time (BT)**, specifically the Ivy or Duke method. Bleeding time measures the time it takes for bleeding to stop after a standardized incision is made on the skin. This test evaluates the body's ability to form a platelet plug and initiate primary hemostasis. It is a functional test that assesses the platelet's ability to aggregate and adhere to the site of injury, which is the primary mechanism of platelet function.

**Why Each Wrong Option is Incorrect**
**Option A:** Platelet Count is a quantitative measure that reflects the number of platelets in the blood but does not assess their functional integrity.

**Option C:** Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT) are coagulation tests that evaluate the intrinsic and extrinsic pathways of coagulation, respectively. While they can indicate the presence of a coagulopathy, they do not directly assess platelet function.

**Clinical Pearl / High-Yield Fact**
Bleeding time is a useful clinical tool for diagnosing platelet disorders, such as Glanzmann's thrombasthenia, Bernard-Soulier syndrome, or aspirin-induced platelet dysfunction. Remember that a prolonged bleeding time can indicate a platelet-related bleeding disorder, while a normal bleeding time does not rule out a platelet disorder.

**Correct Answer: B. Bleeding Time (BT)**