**Core Concept**
Meningococcal meningitis is a severe bacterial infection caused by Neisseria meningitidis, which can be life-threatening if not treated promptly. In pregnancy, the risk of meningococcal meningitis is increased, making prophylaxis crucial to prevent transmission to the fetus and mother.

**Why the Correct Answer is Right**
Ceftriaxone is a third-generation cephalosporin antibiotic that is effective against Neisseria meningitidis. It is the preferred choice for prophylaxis in pregnancy due to its broad-spectrum activity, safety profile, and ability to cross the placenta. Ceftriaxone can be administered intramuscularly, making it a convenient option for pregnant women who may have difficulty swallowing or retaining oral medication.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillin is not the preferred choice for meningococcal meningitis prophylaxis due to concerns about penicillin resistance. While penicillin may still be effective in some cases, ceftriaxone is generally preferred.
**Option C:** Rifampicin is an antibiotic used to treat tuberculosis, not meningococcal meningitis. While it does have some activity against Neisseria meningitidis, it is not the preferred choice for prophylaxis in pregnancy.
**Option D:** Ampicillin is a broad-spectrum penicillin antibiotic, but it is not the first-line choice for meningococcal meningitis prophylaxis in pregnancy. Ceftriaxone is generally preferred due to its broader spectrum of activity and safety profile.

**Clinical Pearl / High-Yield Fact**
In pregnant women, the risk of meningococcal meningitis is highest during the third trimester, making ceftriaxone prophylaxis particularly important during this time.

**✓ Correct Answer: B. Ceftriaxone**