**Core Concept**
The question tests the understanding of adrenergic neurotransmission, specifically the effects of increasing **norepinephrine** concentration in adrenergic synapses. This involves knowledge of **sympathetic nervous system** physiology and **neurotransmitter** regulation.

**Why the Correct Answer is Right**
Increasing **norepinephrine** concentration enhances the stimulation of **adrenergic receptors**, such as **alpha** and **beta** receptors, leading to various physiological effects. This is because **norepinephrine** is the primary neurotransmitter released by the **sympathetic nervous system**, and its increased concentration amplifies the sympathetic response.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because increasing **norepinephrine** does not typically decrease sympathetic activity.
**Option B:** Incorrect as it does not accurately describe the primary effect of increased **norepinephrine** in adrenergic synapses.
**Option C:** Incorrect because the effect of increased **norepinephrine** is more complex than simply decreasing parasympathetic activity.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **norepinephrine** plays a crucial role in the body's **fight-or-flight** response, mediating effects such as increased heart rate and blood pressure.

**Correct Answer:** D. increased sympathetic activity