**Core Concept**
The use of vaccines during pregnancy is a crucial aspect of maternal and fetal health. Vaccines are categorized into live attenuated, inactivated, and toxoid-based types, each with distinct safety profiles for use in pregnant women. The Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG) provide guidelines for vaccine administration during pregnancy.

**Why the Correct Answer is Right**
Inactivated vaccines are generally considered safe for use in pregnant women. These vaccines are made from killed or inactivated pathogens, which cannot cause the disease but still stimulate an immune response. The inactivated flu vaccine is a classic example of an inactivated vaccine used during pregnancy to protect against seasonal influenza. The mechanism of action involves the introduction of inactivated viral particles, which are then processed by antigen-presenting cells and stimulate a T-cell and B-cell response. This leads to the production of antibodies that provide immunity against future infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Live attenuated vaccines contain weakened pathogens, which can pose a risk of transmission to the fetus. Examples include the MMR vaccine and the varicella vaccine.
**Option B:** Toxoid-based vaccines are derived from inactivated toxins, but some components may not be well-studied in pregnant women, making them a less favorable option.
**Option D:** There is limited data on the safety of combination vaccines in pregnant women, and their use is typically reserved for specific circumstances.

**Clinical Pearl / High-Yield Fact**
Pregnant women should receive the inactivated flu vaccine annually to protect against seasonal influenza, which can lead to severe illness and pregnancy complications.

**Correct Answer:** C. Inactivated vaccines can be used in pregnancy in normal circumstances.