**Core Concept**
In cases of blunt abdominal trauma, the patient's hemodynamic status can be compromised due to internal bleeding, leading to hypotension and tachycardia. The next best step in management is to identify and control the source of bleeding.

**Why the Correct Answer is Right**
The patient's tachycardia (heart rate of 120 bpm) and relative hypotension (blood pressure of 100/80 mm Hg) indicate potential internal bleeding. Given that the airway and respiration have been stabilized, the next step is to assess the patient's circulatory status and identify the source of bleeding, which can be achieved through a focused assessment with sonography for trauma (FAST) or a diagnostic peritoneal lavage (DPL). These procedures help in detecting free intraperitoneal fluid, which can indicate internal bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering fluids without assessing the source of bleeding may lead to a false sense of security and delay in identifying and controlling the bleeding source.
**Option B:** Performing a surgical exploration without first assessing the patient's circulatory status and identifying the source of bleeding may not be the most efficient use of resources and can lead to unnecessary surgical intervention.
**Option C:** Transfusing blood products without first assessing the patient's circulatory status and identifying the source of bleeding may not be the most efficient use of resources and can lead to unnecessary transfusions.

**Clinical Pearl / High-Yield Fact**
In cases of blunt abdominal trauma, a FAST or DPL should be performed promptly to identify the source of bleeding, and fluid resuscitation should be guided by the patient's circulatory status.

**Correct Answer:** C. Perform a FAST or DPL to identify the source of bleeding.