**Core Concept**
The question is testing the student's knowledge of common causes of acute pharyngitis in children, specifically in an unimmunized child. **Acute pharyngitis** is an inflammation of the pharynx, often caused by viral or bacterial pathogens.

**Why the Correct Answer is Right**
The most probable causative organism is **Corynebacterium diphtheriae**, which is a Gram-positive rod responsible for **diphtheria**. This bacterium produces a toxin that causes the characteristic membrane to form on the tonsils and pharynx, leading to symptoms such as sore throat, swelling of the neck, and respiratory distress. The child's unimmunized status increases the risk of diphtheria, making it a likely diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus pyogenes is a common cause of acute pharyngitis in children, but it is more likely to cause a sore throat without the characteristic membrane formation seen in diphtheria.
* **Option B:** Haemophilus influenzae type b (Hib) can cause a range of infections, including epiglottitis and meningitis, but it is not typically associated with pharyngitis.
* **Option D:** Group A beta-hemolytic Streptococcus (GABHS) is another common cause of acute pharyngitis, but it is not the most likely cause in an unimmunized child with a characteristic membrane.

**Clinical Pearl / High-Yield Fact**
Diphtheria is a vaccine-preventable disease, and timely immunization is essential to prevent outbreaks. Children who are unimmunized or under-immunized are at increased risk of contracting diphtheria.

**Correct Answer:** C. Corynebacterium diphtheriae