**Core Concept**
The treatment of bacterial meningitis requires prompt initiation of antibiotics to prevent complications and improve outcomes. The choice of antibiotic depends on the causative pathogen, its antimicrobial susceptibility, and the patient's age, weight, and underlying medical conditions.

**Why the Correct Answer is Right**
For bacterial meningitis caused by Streptococcus pneumoniae, the drug of choice is ceftriaxone. This third-generation cephalosporin has a broad spectrum of activity, including excellent coverage against S. pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, the most common causes of bacterial meningitis in children. Ceftriaxone's long half-life allows for once-daily dosing, which simplifies treatment and improves patient compliance.

**Why Each Wrong Option is Incorrect**
* **Option A:** Penicillin G is effective against S. pneumoniae but may not provide adequate coverage against other pathogens, such as H. influenzae and N. meningitidis. Moreover, penicillin-resistant strains of S. pneumoniae have been reported.
* **Option B:** Vancomycin is a glycopeptide antibiotic that is effective against methicillin-resistant Staphylococcus aureus (MRSA) and other resistant gram-positive bacteria. However, it is not the first-line treatment for bacterial meningitis caused by S. pneumoniae.
* **Option D:** Meropenem is a carbapenem antibiotic with broad-spectrum activity, including against gram-negative bacteria. However, it is generally reserved for patients with severe infections or those who have failed treatment with other antibiotics.

**Clinical Pearl / High-Yield Fact**
When treating bacterial meningitis, it is essential to initiate antibiotics promptly, ideally within 30 minutes of diagnosis, to reduce the risk of complications and improve outcomes. Additionally, a third-generation cephalosporin, such as ceftriaxone, is the preferred choice for empirical therapy until culture and sensitivity results are available.

**Correct Answer:** C. Ceftriaxone.