**Core Concept**
During the last trimester of pregnancy, the cardiovascular system undergoes several physiological changes to accommodate the increased blood volume and cardiac output required to support the growing fetus and prepare for delivery. One of these changes is a reduction in systemic vascular resistance (SVR) and an increase in cardiac output, which is mediated by various hormonal and neural mechanisms.

**Why the Correct Answer is Right**
The correct answer is physiological vasodilation. This is achieved through the decrease in systemic vascular resistance (SVR) due to the vasodilatory effects of progesterone, prostaglandins, and other hormones. The decrease in SVR results in a decrease in peripheral resistance, which in turn increases cardiac output. This increase in cardiac output is necessary to meet the increased metabolic demands of the fetus and the mother. The heart rate also increases during pregnancy, which further contributes to the increase in cardiac output.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a recognized cardiovascular change during pregnancy.
* **Option B:** This option is incorrect as the increase in heart rate is physiological but not the primary change in the last trimester.
* **Option C:** This option is not a recognized cardiovascular change during pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cardiovascular changes during pregnancy are physiological adaptations that allow the body to meet the increased demands of the growing fetus. These changes are essential for a healthy pregnancy outcome and should not be confused with pathological conditions that may require medical intervention.

**Correct Answer: C. Physiological vasodilation**