**Core Concept**
During pregnancy, there is a significant increase in insulin resistance due to various hormonal changes. The pancreas produces more insulin to compensate for this resistance. However, after delivery, the insulin resistance decreases, and insulin sensitivity increases. This is partly due to the decrease in the levels of certain hormones produced during pregnancy.

**Why the Correct Answer is Right**
Plasma chorionic somatomammotropin (hCS or human placental lactogen) is a hormone produced by the placenta during pregnancy. It plays a crucial role in maintaining insulin resistance by interfering with the action of insulin at the insulin receptors. After delivery, the levels of hCS decrease, leading to an increase in insulin sensitivity. This decrease in insulin resistance results in a decrease in insulin requirements in diabetic patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased food intake may actually increase insulin requirements, not decrease them. While food intake can affect blood sugar levels, it is not the primary reason for the decrease in insulin requirements postpartum.
**Option B:** Decreased activity may have a minor effect on insulin requirements, but it is not the primary reason for the decrease in insulin requirements postpartum.
**Option D:** All of the above options are not correct. While they may have some effect on insulin requirements, they are not the primary reason for the decrease in insulin requirements postpartum.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor blood sugar levels closely in diabetic patients postpartum, as the decrease in insulin resistance can lead to an increase in blood sugar levels. This is particularly important in patients with pre-existing diabetes or those who developed gestational diabetes during pregnancy.

**✓ Correct Answer: C. Decrease in plasma chorionic somatomammotropin**