**Core Concept**
Live attenuated vaccines can pose a risk to the fetus by causing an active infection. Therefore, they are contraindicated in pregnancy.

**Why the Correct Answer is Right**
The correct answer is a live attenuated vaccine. Live attenuated vaccines contain a weakened form of the virus or bacteria, which can cause an active infection in the fetus. This is particularly concerning during pregnancy when the immune system is suppressed. The use of live attenuated vaccines during pregnancy has been associated with an increased risk of congenital abnormalities and other adverse outcomes. The most notable example is the live attenuated rubella vaccine.

**Why Each Wrong Option is Incorrect**
**Option A:** Inactivated vaccines are generally safe to use during pregnancy and are often recommended to protect against diseases such as flu and pertussis. These vaccines do not contain live viruses and cannot cause an infection in the fetus.
**Option B:** Toxoids are also safe to use during pregnancy and are used to protect against diseases such as tetanus. Toxoids are inactivated toxins that cannot cause an infection in the fetus.
**Option C:** Conjugate vaccines are also safe to use during pregnancy and are used to protect against diseases such as pneumococcal disease. Conjugate vaccines are made by attaching a polysaccharide antigen to a protein, which makes them more effective and safer to use during pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while live attenuated vaccines are contraindicated during pregnancy, inactivated vaccines and toxoids are generally safe to use. This highlights the importance of understanding the differences between various vaccine types and their safety profiles during pregnancy.

**Correct Answer:** D. Live attenuated vaccines.