**Core Concept:** Viral pneumonia is an infection in the lungs caused by viral pathogens, which can lead to inflammation and breathing difficulties in infants. The answer focuses on identifying the most common cause of viral pneumonia in infants.

**Why the Correct Answer is Right:** The correct answer, **C. Influenza Virus**, is the leading cause of viral pneumonia in infants due to its high prevalence and the severity of the disease it can cause. Influenza virus infections primarily target the respiratory system, leading to inflammation and subsequent pneumonia.

**Why Each Wrong Option is Incorrect:**
A. **RSV (Respiratory Syncytial Virus)** is also a common cause of viral pneumonia in infants, but **D. Influenza Virus** is more likely to cause severe disease and is responsible for a higher number of hospitalizations and deaths among infants as compared to RSV.
B. **Human Herpesvirus (HHV)**, also known as Cytomegalovirus (CMV), can cause viral pneumonia in infants but is **not the most common cause**.
D. **Parainfluenza Virus** can cause viral pneumonia in infants, but it is **less common than Influenza Virus**.

**Clinical Pearl:** In clinical practice, distinguishing between RSV and Influenza virus infections in infants presenting with respiratory symptoms is crucial, as they require different management approaches.

**Correct Answer:** C. Influenza Virus