**Core Concept**
The underlying principle in this question involves the understanding of the pharmacodynamics of norepinephrine and atropine. Norepinephrine is an adrenergic agonist that has both alpha and beta-adrenergic effects, which can lead to various cardiovascular responses. Atropine, on the other hand, is an anticholinergic agent that blocks the action of acetylcholine at muscarinic receptors, resulting in increased heart rate and decreased parasympathetic tone.

**Why the Correct Answer is Right**
When norepinephrine is administered intravenously to a patient already on atropine, it will cause tachycardia due to its beta-adrenergic effects. The beta-1 adrenergic receptors in the heart are stimulated, leading to increased heart rate and contractility. However, atropine has already blocked the parasympathetic effects of acetylcholine, making the heart rate more susceptible to the sympathetic effects of norepinephrine. This results in a pronounced increase in heart rate, which is a characteristic effect of combined administration of norepinephrine and atropine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we will start with **Option B**.
* **Option B:** This option is also not provided, so we will move on to **Option C**.
* **Option D:** This option is not relevant to the scenario described in the question.

**Clinical Pearl / High-Yield Fact**
When administering norepinephrine to patients on atropine, it is essential to monitor their heart rate closely, as the combination can lead to significant tachycardia.

**Correct Answer:** C. Tachycardia.