**Core Concept**
The underlying principle being tested is the diagnosis of recurrent hypoglycemia, specifically focusing on the role of **insulin** and **catecholamines** in hypoglycemic episodes. The patient's symptoms, such as shakiness, anxiety, and sweating, are classic for hypoglycemia, which can be caused by various factors including excess insulin.

**Why the Correct Answer is Right**
Given the patient's symptoms and the fact that episodes only occur at work, where she has access to insulin as a nurse, the most likely cause is **insulin-induced hypoglycemia**. Drinking coffee, which contains **caffeine**, a stimulant, could temporarily alleviate symptoms by increasing glucose release from energy stores and reducing insulin sensitivity. The appropriate test would be one that detects evidence of excess insulin during episodes of hypoglycemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it suggested a test for a condition not directly related to the suspected cause of hypoglycemia in this context.
**Option B:** Similarly, if it does not directly address the potential for insulin-induced hypoglycemia.
**Option C:** Might be incorrect if it pertains to a test that does not specifically diagnose the cause of hypoglycemia in the context provided.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that hypoglycemic episodes caused by excess insulin can be diagnosed by measuring **C-peptide** levels during an episode, as this will be elevated if the hypoglycemia is due to exogenous insulin administration.

**Correct Answer:** D. C-peptide levels during hypoglycemia.