**Core Concept**
The clinical presentation suggests a condition affecting the hip or surrounding structures, such as **transient synovitis** or **slipped capital femoral epiphysis**, which can cause hip pain and limp in a child. The **femoral triangle** tenderness is a significant finding.

**Why the Correct Answer is Right**
Since the X-ray is normal, further imaging is needed to rule out conditions not visible on plain X-ray, such as **transient synovitis** or early **Perthes disease**. An **ultrasound** or **MRI** can be helpful, but in the context of suspected hip pathology in a child with a normal X-ray, the next best step often involves assessing for signs of infection or inflammation and possibly performing an **ultrasound** to evaluate the hip joint for effusion, which is a common finding in **transient synovitis**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without more context, it's hard to judge, but typically, immediate action without further diagnosis is not ideal.
**Option B:** Similarly, without context, it's challenging to assess its appropriateness.
**Option D:** This option is also not provided, making it impossible to evaluate.

**Clinical Pearl / High-Yield Fact**
In children presenting with hip pain and limp, it's crucial to consider **slipped capital femoral epiphysis** and **Perthes disease** early, as prompt diagnosis can significantly impact treatment and outcomes.

**Correct Answer:** D. Ultrasound of the hip.