Final answer:
The increase in heart rate when moving to a standing position is primarily due to sympathetic activity that stimulates the heart to beat faster and constrict blood vessels to maintain blood flow to the brain.
Step-by-step explanation:
One explanation for an increase in heart rate when moving from a lying to a standing position is C) Sympathetic activity. When you stand up, gravity causes more blood to flow into your legs, which could reduce the amount of blood returning to the heart and subsequently to the brain. The body responds to this through a sympathetic reflex; the sympathetic nervous system increases heart rate and constricts blood vessels to maintain blood flow, especially to the brain. Venoconstriction, caused by sympathetic stimulation, thus increases venous return to the heart, as well as blood pressure in the veins and blood flow, which all contribute to the heart beating faster to maintain adequate blood delivery to our organs.
Another factor, venous return, is increased by the constriction of veins, enhancing the flow of blood back to the heart. However, the primary immediate adjustment to the postural change is handling by the sympathetic nervous system's activation to maintain blood pressure and flow to vital organs, especially the brain.