**Core Concept**
Toxoplasmosis is a parasitic infection caused by *Toxoplasma gondii*, which can be transmitted to humans through various routes, including vertical transmission from mother to fetus during pregnancy. The risk of transmission depends on the gestational age at the time of infection.

**Why the Correct Answer is Right**
The highest rate of transmission of toxoplasmosis during pregnancy occurs when the mother is infected during the first trimester, particularly between weeks 8 and 12. This is because the parasite crosses the placenta and infects the fetus, causing congenital toxoplasmosis. The severity of the infection depends on the gestational age at the time of maternal infection, with first-trimester infections being associated with the highest risk of severe outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as while toxoplasmosis can be transmitted during the second and third trimesters, the rate of transmission is lower compared to the first trimester.
**Option B:** Incorrect, as there is no specific evidence to suggest that toxoplasmosis transmission rates vary significantly by trimester in the general population.
**Option C:** Incorrect, as this option does not provide a clear trimester-specific risk of toxoplasmosis transmission during pregnancy.

**Clinical Pearl / High-Yield Fact**
It is essential for pregnant women to avoid undercooked meat, unwashed vegetables, and cat feces to reduce the risk of toxoplasmosis transmission. Pregnant women should also be tested for toxoplasmosis if they have a history of cat ownership or have recently consumed undercooked meat.

**Correct Answer: D. First trimester.**