**Core Concept**
Recommended Dietary Allowance (RDA) in pregnancy is increased to meet the nutritional needs of the mother and the developing fetus. This includes calories, essential micronutrients, and macronutrients to support fetal growth, maternal health, and lactation.

**Why the Correct Answer is Right**
The RDA for calories is increased by 300-350 kcal/day during the second and third trimesters to support fetal growth and maternal energy expenditure. The increased demand for iron is due to the expansion of maternal red blood cell mass and increased fetal requirements. Folic acid is essential for preventing neural tube defects and other congenital abnormalities. The RDA for folic acid is increased to 600-800 μg/day, but 4 mg is mentioned, which is a safe upper limit for pregnant women. The RDA for magnesium is 310-320 mg/day, but 2500 mg is excessive and potentially toxic. Iodine is crucial for thyroid hormone production and fetal brain development, but the recommended intake is 220 μg/day, not 2500 mg, which is an overdose.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase of 100-300 kcal is incorrect because the actual increase is 300-350 kcal/day.
**Option B:** The RDA for iron is increased to 9-12 mg/day, not 30 mg.
**Option C:** The correct RDA for folic acid is 600-800 μg/day, but 4 mg is a safe upper limit.
**Option D:** 2500 mg is an excessive dose of magnesium, which can cause toxicity.
**Option E:** 2500 mg is an overdose of iodine, which can cause thyroid problems.

**Clinical Pearl / High-Yield Fact**
The RDA for calories and essential micronutrients increases during pregnancy to support maternal and fetal health. Pregnant women should consult their healthcare providers to determine their individual nutritional needs.

**Correct Answer: C. 4 mg folic acid**