Final answer:
The preganglionic sympathetic fibers projecting to the heart are called cardiac accelerator nerves; they are responsible for increasing the heart rate with sympathetic stimulation.
Step-by-step explanation:
The correct answer to the question is D) cardiac accelerator nerve. Preganglionic sympathetic fibers that project to the heart are known as cardiac accelerator nerves. These nerves originate from the cardiovascular center in the medulla and pass through sympathetic ganglia to stimulate the heart at the SA and AV nodes, as well as the atria and ventricles. When activated by the body's sympathetic nervous system, they increase the heart rate by causing the release of the neurotransmitter norepinephrine, which in turn increases ion flow and accelerates depolarization and contraction of the cardiac muscle cells.