The main effect of the sympathetic system in relation to heart rate is to:

The sympathetic nervous system plays a crucial role in the body's response to stress, often referred to as the "fight or flight" response. When the sympathetic system is activated, it releases neurotransmitters like norepinephrine that bind to adrenergic receptors in the heart. This binding has the effect of increasing heart rate by enhancing the rate of depolarization in the sinoatrial (SA) node, which is responsible for initiating heartbeats.

Additionally, the sympathetic activation not only increases the heart rate but also boosts the force of heart contractions, enabling the heart to pump more blood effectively during periods of high demand, such as during exercise or stress. This heightened state of readiness is essential for responding to immediate challenges or threats, making the increase in heart rate a vital mechanism in the overall function of the cardiovascular system.

The other choices do not accurately reflect the primary role of the sympathetic nervous system in heart rate regulation. Reducing, stabilizing, or normalizing heart rate would align more closely with the functions of the parasympathetic system, which is responsible for promoting a state of rest and digestion, thereby lowering heart rate and conserving energy.