**Core Concept**
During pregnancy, the recommended dietary allowance (RDA) increases to meet the growing demands of the fetus, placenta, and mother. Adequate nutrition is crucial for fetal growth and development, as well as maternal health. One essential micronutrient is iron, which plays a critical role in preventing anemia and supporting the increased oxygen-carrying capacity of the blood.

**Why the Correct Answer is Right**
The RDA for iron during pregnancy is indeed 35 mg/day, as recommended by various health organizations, including the World Health Organization (WHO) and the National Academy of Sciences (NAS). This increase is necessary to compensate for the expansion of maternal red blood cell mass, increased blood volume, and higher fetal requirements. Iron deficiency anemia is a common complication of pregnancy, and adequate iron intake helps prevent this condition. The enzyme heme oxygenase, which breaks down heme in red blood cells, is also affected by iron status, highlighting the importance of iron in maintaining healthy red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** The recommended caloric increase during pregnancy is typically around 300-500 calories/day, not 100-300 calories. This increase supports the energy needs of the growing fetus and mother.
**Option C:** While folic acid is crucial for fetal development, particularly in preventing neural tube defects, the recommended dose during pregnancy is 600-800 μg/day, not 4 mg.
**Option D:** The recommended magnesium intake during pregnancy is around 310-320 mg/day, not 2500 mg.

**Clinical Pearl / High-Yield Fact**
The "March of Dimes" recommends that all pregnant women take a daily supplement containing 400-800 μg of folic acid to prevent neural tube defects.

**✓ Correct Answer: B. 35 mg Iron**