**Core Concept**
The additional daily energy requirement for a lactating woman is crucial for milk production and maternal health. This increase is due to the energy expended in producing milk, which is estimated to be around 500-600 kcal/day. The energy requirement is also influenced by the mother's pre-pregnancy body mass index (BMI) and the frequency of breastfeeding.

**Why the Correct Answer is Right**
The energy expenditure for lactation is primarily due to the breakdown of fatty acids in the adipose tissue to produce milk. This process is mediated by the hormone prolactin, which stimulates the lipolysis of fat cells. The energy from these broken-down fatty acids is then used to synthesize milk, which is composed of water, carbohydrates, proteins, and fats. The additional energy requirement is also influenced by the caloric content of the milk produced, which is around 70 kcal/100 ml for breast milk.

**Why Each Wrong Option is Incorrect**
**Option A:** A 200 kcal/day increase is too low to meet the energy needs for lactation.
**Option B:** A 1,200 kcal/day increase is excessive and not supported by scientific evidence.
**Option C:** A 400 kcal/day increase is closer, but still lower than the recommended value.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the additional energy requirement for lactation is higher in the first six months, and the mother's diet should be tailored to meet these increased energy needs to support milk production and maternal health.

**Correct Answer: C. 500-600 kcal/day.**